Chapter 5. Parathyroid Gland and Ca²⁺ and PO₄^- Regulation

• Identify the origin, target organs and cell types, and physiologic effects of parathyroid hormone.
• Describe the functions of osteoblasts and osteoclasts in bone remodeling and the factors that regulate their activities.
• Describe the regulation of parathyroid hormone secretion and the role of the calcium-sensing receptor.
• Identify the sources of vitamin D and describe the biosynthetic pathway involved in modifying it to its biologically active form.
• Identify the target organs and cellular mechanisms of action of vitamin D.
• Describe the negative feedback relationship between parathyroid hormone and the biologically active form of vitamin D.
• Describe the causes and consequences of excess or deficiency of parathyroid hormone and of vitamin D.
• Describe the regulation of calcitonin release and the cell of origin and target organs for calcitonin action.

The regulation of plasma Ca2+ levels is critical for normal cell function, neural transmission, membrane stability, bone structure, blood coagulation, and intracellular signaling. This regulation relies on the interactions among parathyroid hormone (PTH) from the parathyroid glands, dietary vitamin D, and calcitonin from the thyroid gland (Figure 5–1). PTH stimulates bone resorption and the release of Ca2+ into the circulation. In the kidney, PTH promotes Ca2+ reabsorption and inorganic phosphate excretion in the urine. In addition, PTH stimulates the hydroxylation of 25-hydroxyvitamin D at the 1-position, leading to the formation of the active form of vitamin D (calcitriol). Vitamin D increases intestinal absorption of dietary Ca2+ and facilitates renal reabsorption of filtered Ca2+. In bone, vitamin D increases bone resorption with a resulting increase in the release of Ca2+ into the circulation. Calcitonin counteracts the effects of PTH by inhibiting bone resorption and increasing renal Ca2+ excretion. The overall result of the interactions among PTH, vitamin D, and calcitonin is the maintenance of normal plasma Ca2+ concentrations.

The parathyroid glands are pea-sized glands located at the top and bottom posterior borders of the lateral lobes of the thyroid gland. The glands are richly vascularized and consist primarily of chief cells, with a thin capsule of connective tissue that divides the gland into lobules. The chief cells synthesize and secrete PTH, a polypeptide hormone that plays a major role in bone remodeling and calcium homeostasis. In addition to its central role in the regulation of Ca2+ levels and bone mass, PTH participates in the renal excretion of phosphate and in the activation of vitamin D. Because of the central role of PTH in calcium homeostasis, its physiologic effects are described in the context of its interaction with the other major regulators of calcium: calcitonin, the hormone synthesized in the C cells of the thyroid gland; and calcitriol, the active form of vitamin D.

PTH is synthesized as a prepropeptide that is rapidly cleaved to yield pro-PTH and subsequently the mature form of PTH. Mature and intact PTH consists of 84 amino acids. PTH synthesis and release is continuous, with about 6–7 superimposed pulses each hour. PTH is degraded by the kidney and the liver to amino-terminal (PTH 1-34) and carboxy-terminal fragments. The amino-terminal fragments constitute approximately 10% of circulating PTH fragments. They are biologically active but have a short half-life (4–20 minutes). In contrast, the carboxy-terminal fragments, which constitute 80% of the circulating PTH fragments, have no biologic activity and have a longer half-life; they are therefore easier to measure in the plasma. Overall, the intact hormone accounts for 10% of the circulating PTH-related peptides. Because multiple peptide products of PTH breakdown are present in the circulation, determination of the intact molecule is the only reliable index of PTH levels.

Regulation of Parathyroid Hormone Release

Parathyroid Ca2+ Receptor

The Ca2+ sensor is a G protein (Gq/11 and Gi)–coupled receptor located on the plasma membrane of the parathyroid chief cells; it is also found in kidney tubule cells and thyroid C cells. The release of PTH is inhibited in response to elevations in plasma Ca2+ concentrations and activation of the Ca2+ receptor (see Figure 5–2). Activation of the Ca2+ receptor initiates a signaling cascade involving phospholipases C, D, and A2. The phosphorylation and activation of phospholipase A2 activate the arachidonic acid cascade and increase leukotriene synthesis. The active leukotriene metabolites inhibit PTH secretion. The inhibition of PTH secretion by elevated Ca2+ levels is the result of increased degradation of preformed hormone stored in secretory granules. The granules are under regulatory control by plasma Ca2+ levels; persistent hypercalcemia leads to rapid degradation of most (90%) of the mature PTH in the cell. This process results in the formation of inactive carboxy-terminal PTH fragments, which are either released into the circulation or further degraded in the parathyroid. The amino acids released during degradation of the formed PTH inside the parathyroid cells are reused in the synthesis of other proteins. In contrast, vitamin D [1,25(OH)2D] inhibits PTH release by decreasing PTH gene expression.

During hypocalcemia, the parathyroid Ca2+ receptor is relaxed, and PTH secretion is not restrained. The rapid secretion of preformed PTH elicited by acute hypocalcemia is rapidly followed by increased stability of PTH mRNA and the synthesis of new hormone.

Additional Factors that Regulate Parathyroid Hormone Release

PTH release is also under regulatory control by phosphate and magnesium levels. Elevations in plasma phosphate levels increase PTH secretion by decreasing phospholipase A2 activity and arachidonic acid formation, thus removing the inhibitory effect on PTH secretion. Elevations in phosphate levels can also affect PTH release indirectly by decreasing plasma Ca2+ levels and vitamin D activation. In contrast, hypophosphatemia markedly decreases PTH mRNA and plasma PTH. The role of phosphate in PTH release is critical in patients with impaired renal function. If not controlled, the increasing levels of phosphate in these individuals result in abnormal elevations in PTH release. As the disease progresses, the negative feedback regulation of PTH release by Ca2+ and vitamin D is also impaired, contributing to the elevation in PTH release. As explained in the following section, chronic elevation of PTH results in excess bone resorption and loss of bone mass.

Plasma Mg2+ concentrations also regulate PTH secretion in a similar manner to that of calcium. PTH release can be stimulated by a moderate decrease in plasma Mg2+. However, very low serum concentrations of Mg2+ induce a paradoxical block in PTH release. The mechanism for this effect has been suggested to be the result of activation of the α-subunits of the calcium-sensing receptor G-proteins, mimicking the activation of the receptor and thus inhibiting PTH secretion.

Plasma magnesium levels reflect those of calcium most of the time. The balance of magnesium is closely linked to that of calcium. Magnesium depletion or deficiency is frequently associated with hypocalcemia. This combined decrease in Mg2+ and Ca2+ leads to impairment in the individual’s ability to secrete PTH. Moreover, severe hypomagnesemia impairs not only the release of PTH from the parathyroid gland in response to hypocalcemia, but it also prevents the responsiveness of bone to PTH-mediated bone resorption. Adrenergic agonists have been shown to increase PTH release through β-adrenergic receptors on parathyroid cells.

Parathyroid Hormone Receptor 1

PTH binding to the G protein–coupled PTHR1 initiates a cascade of intracellular processes primarily by signaling through the α-subunit of the stimulatory G-protein (Gαs) leading to an increased synthesis of cyclic 3',5'-adenosine monophosphate (cAMP) and activation of protein kinase A and phosphorylation of target proteins at serine residues. The result is the activation of preformed proteins, as well as the induction of gene transcription. In addition, activation of the PTHR1 can use additional signaling pathways through Gαq leading to the activation of phospholipase C, increased intracellular inositol 1, 4, 5-trisphosphate and calcium concentrations.

Cellular Effects of Parathyroid Hormone

Inside the tubular epithelial cell, Ca2+ binds to calbindin-D28K and then diffuses out through the basolateral membrane (see Figure 5–3). Calbindin-D28K is a vitamin D–dependent calcium-binding protein that is present in the cytosol of cells lining the distal part of the nephron. Calbindin facilitates the cytosolic diffusion of Ca2+ from the apical influx to the basolateral efflux sites. It is thought to act as either a transport protein or a buffer to prevent excess elevation of cytosolic Ca2+ levels. Transport of Ca2+ out of the cell into the interstitial space is mediated by a Na+/Ca2+ exchanger and a Ca2+-adenosine triphosphatase (ATPase). A similar mechanism to aid in Ca2+ absorption is found in intestinal epithelial cells.

Parathyroid Hormone Mobilization of Bone Ca2+

Osteoblasts

Osteoblasts express PTH receptors and are responsible for bone formation and mineralization. They are derived from pluripotent mesenchymal stem cells, which can also differentiate into chondrocytes, adipocytes, myoblasts, and fibroblasts. Several hormonal and nonhormonal molecules stimulate the differentiation of osteoblasts from stem cell precursors.

Osteoclasts

Osteoclasts are large multinucleated bone-resorbing cells derived from hematopoietic precursors of the monocyte-macrophage lineage. They are formed by the fusion of mononuclear cells and are characterized by a ruffled border, which consists of an infolding of the plasma membrane, and a prominent cytoskeleton. Osteoclasts are rich in lysosomal enzymes.

Osteocytes

Osteocytes, the most numerous cells in bone, are small, flattened cells within the bone matrix. They are connected to one another and to osteoblastic cells on the bone surface by an extensive canalicular network that contains the bone’s ECF. These cells are terminally differentiated from osteoblasts and ultimately undergo apoptosis or phagocytosis during osteoclastic resorption. Their function and their contribution to osteoclastic bone resorption are not completely understood.

Mechanism of Bone Resorption

Bone remodeling involves the continuous removal of bone (bone resorption) followed by synthesis of new bone matrix and subsequent mineralization (bone formation). This coupling of the functions of osteoblasts and osteoclasts is central not only for the hormonal regulation of bone turnover, but also for the evaluation of their altered function, as discussed later. During skeletal development and throughout life, cells from the osteoblast lineage synthesize and secrete molecules that in turn initiate and control osteoclast differentiation. Osteoclasts are stimulated hormonally and locally by growth factors and cytokines. Osteoclastic activity induced by PTH is indirectly mediated through osteoblast activation. Osteoclastic bone resorption consists of multiple steps, including recruitment and differentiation of osteoclast precursors into mononuclear osteoclasts (preosteoclasts) and fusion of preosteoclasts to form multinucleated functional osteoclasts (Figure 5–6).

The initial event in bone degradation is the attachment of osteoclasts to the bone surface following their recruitment. PTH binding to osteoblasts triggers the synthesis of ODF, also known as RANKL or osteoprotegerin ligand. PTH stimulates the expression of this cell membrane protein in osteoblastic cells. This ligand binds to the ODF receptor (receptor activator of nuclear factor-κB [RANK]) expressed on the hemopoietic osteoclastic precursors and stimulates their differentiation into functional osteoclasts. These 2 cell surface proteins, RANK expressed on osteoclast precursor cells and its partner RANKL expressed on osteoblasts, are the key regulators of osteoclast formation and function (see Figures 5–5 and 5–6).

Activation by RANKL increases the expression of specific genes leading to osteoclast maturation. When an osteoclast precursor encounters an osteoblast, the resulting interaction between RANK and RANKL stimulates the osteoclast precursor to mature into a fully differentiated, bone-resorbing osteoclast. Additional proteins play important roles in the regulation of this mechanism. Osteoprotegerin is a protein member of the tumor necrosis factor (TNF) receptor superfamily, secreted by osteoblasts which acts as a natural antagonist of RANKL and thus contributes to the regulation of bone resorption. Osteoprotegerin functions as an osteoclastogenesis inhibitory factor. It works as a soluble “decoy” protein that binds to RANKL, preventing it from binding to RANK and thereby effectively inhibiting RANKL-mediated osteoclast maturation. The regulation of osteoprotegerin production is not completely understood. However, it is known that PTH and glucocorticoids decrease the production of osteoprotegerin, whereas estrogens increase its expression.

Osteoclastic bone resorption involves the attachment of osteoclasts by proteins called β-integrins to the bone surface, generating an isolated extracellular microenvironment between osteoclasts and the bone surface that in effect functions as a lysosome. Acidic intracellular vesicles fuse to the ruffled border of the cell membrane facing the bone matrix—the hallmark of the resorbing surface—under which bone resorption takes place. Acidification of this extracellular microenvironment is mediated by H+-ATPases recruited into the cell’s ruffled membrane. During bone resorption, hydrogen ions generated by carbonic anhydrase II are delivered across the plasma membrane by a proton pump to dissolve bone mineral. The decrease in pH (pH 4) in the sealed compartment favors the dissolving of hydroxyapatite bone mineral and provides optimal conditions for the action of the lysosomal proteases, including collagenase and cathepsins secreted by osteoclasts. The products of bone degradation (including Ca2 and phosphate), are endocytosed by the osteoclast and released at the cell’s antiresorptive surface by a process called transcytosis (transit across the cell), from where they then reach the systemic circulation. During bone degradation, intracellular enzymes such as alkaline phosphatase are also released into the interstitial space to enter the circulation. An increase in circulating levels of alkaline phosphatase can be used clinically as a marker of increased osteoclastic activity.

The human body contains approximately 1100 g of calcium, 99% of which is deposited in bones and teeth. The small amount found in plasma is divided into 3 fractions: ionized calcium (50%), protein-bound calcium (40%), and calcium complexed to citrate and phosphate forming soluble complexes (10%) (Figure 5–7). The complexed and ionized Ca2+ fractions (approximately 60% of plasma Ca2+) can cross the plasma membrane. The majority (80%–90%) of protein-bound Ca2+ is bound to albumin, and this interaction is sensitive to changes in blood pH. Acidosis leads to a decrease in protein binding of Ca2+ and an increase in “free” or ionized Ca2+ in the plasma. Alkalosis results in increased Ca2+ binding and a decrease in ionized Ca2+ in the plasma. A smaller fraction (10%–20%) of protein-bound Ca2+ is bound to globulins.

The resting intracellular (cytosolic) calcium concentration is approximately 100 nM, but this can increase to 1 μM by the release of Ca2+ from intracellular stores or by uptake of extracellular Ca2+ in response to cellular activation. In contrast, the extracellular ionized calcium concentration is approximately 10,000-fold higher than the intracellular calcium concentration and remains virtually constant at approximately 1 mM. Multiple physiologic functions involve calcium ions. Ca2+ is a key intracellular messenger and cofactor for various enzymes. Ca2+ also has diverse extracellular functions (eg, in the clotting of blood, maintenance of skeletal integrity, and modulation of neuromuscular excitability). Therefore, stable Ca2+ levels are critical for normal physiologic function. For example, Na+ channel voltage-gating is dependent on the extracellular Ca2+ concentration. Decreased plasma Ca2+ concentrations reduce the voltage threshold for the action potential firing, resulting in neuromuscular hyperexcitability. This can result in numbness and tingling of fingertips, toes, and the perioral region or muscle cramps. Clinically, neuromuscular irritability can be demonstrated by mechanical stimulation of the hyperexcitable nerve leading to tetanic-like muscle contraction by eliciting Chvostek (ipsilateral contraction of facial muscles elicited by tapping the skin over the facial nerve) or Trousseau (carpal spasm induced by inflation of the blood pressure cuff to 20 mm Hg above the patient’s systolic blood pressure for 3–5 minutes) sign.

Interaction of Bone, Kidney and Intestine in Maintaining Calcium Homeostasis

Normal plasma concentrations of Ca2+ range between 8.5 and 10.5 mg/dL and are mainly regulated by the actions of PTH, vitamin D, and calcitonin on 3 tissues: bone, kidney, and intestine.

Bone

Calcium in bone is distributed in a readily exchangeable pool and a stable pool. The readily exchangeable pool is involved in maintaining Ca2+ plasma levels by the daily exchange of 550 mg of calcium between the bone and ECF. The stable Ca2+ pool is involved in bone remodeling. Bone is metabolically active throughout life. After skeletal growth is complete, remodeling of both cortical and trabecular bone continues with an annual turnover rate of approximately 10% of the adult skeleton.

Kidney

In the kidney, virtually all filtered Ca2+ is reabsorbed. Approximately 40% of the Ca2+ that is reabsorbed is under hormonal regulation by PTH. Most filtered Ca2+ is reabsorbed in the proximal tubules, mainly by passive transport processes independent of hormonal regulation. Ca2+ absorption in the cortical thick ascending limbs is mediated by a combination of active and passive absorption. Ca2+ absorption in the distal convoluted tubules is mediated by active cellular absorption, which is stimulated by PTH binding to PTHR1. Transcellular transport of Ca2+ is facilitated by vitamin D through the increase in the Ca2+-binding protein calbindin-D28K and in the expression of Ca2+ transporters in the basolateral membrane.

Intestine

The availability of dietary calcium is a critical determinant of calcium homeostasis. Dietary intake of calcium averages 1000 mg/d, of which only 30% is absorbed in the intestinal tract (Figure 5–8). This percentage of dietary Ca2+ that is absorbed is significantly enhanced by vitamin D during growth, pregnancy, and lactation. During growth, there is a net bone accretion. After completion of the growth phase in the young and healthy individual, there is no net gain or loss of Ca2+ from bone despite a continuous turnover of bone mass; the amount of Ca2+ lost in urine is approximately equal to net Ca2+ absorption.

Intestinal absorption of Ca2+ occurs by a saturable, transcellular process and a nonsaturable, paracellular pathway. The paracellular pathway predominates when dietary Ca2+ is abundant. The active transcellular pathway is vitamin D–dependent and plays a major role in absorption when the Ca2+ supply is limited. Intestinal transepithelial Ca2+ transport, similar to that in the distal tubule, is a 3-step process consisting of passive entry across the apical membrane, cytosolic diffusion facilitated by vitamin D–dependent calcium-binding proteins (calbindins), and active extrusion of Ca2+ across the opposing basolateral membrane mediated by a high-affinity Ca2+-ATPase and Na+/Ca2+ exchanger.

Hormonal Regulation of Calcium Homeostasis

As explained previously, a slight decrease in the free ionized Ca2+ level is sensed through the Ca2+ sensor in parathyroid chief cells, resulting in an increased release of PTH. PTH binds to receptors in osteoblasts leading to the recruitment of preosteoclasts and their maturation to active osteoclasts, which are responsible for increased bone resorption and release Ca2+ and inorganic phosphate (Pi) into the circulation. In the kidney, PTH promotes Ca2+ reabsorption and Pi excretion in urine. In addition, PTH stimulates the hydroxylation of 25-hydroxyvitamin D3 at the 1-position, leading to the formation of the active form of vitamin D (calcitriol). Vitamin D increases intestinal absorption of dietary Ca2+ and renal reabsorption of filtered Ca2+. In bone, vitamin D increases the number of osteoclasts and stimulates bone resorption, with a resulting increase in the release of Ca2+ into the circulation. An increase in free ionized Ca2+ levels decreases the release of PTH from the parathyroid gland, decreases the activation of vitamin D in the kidney, and stimulates parafollicular cells of the thyroid gland to release the hormone calcitonin.

Role of Vitamin D in Calcium Homeostasis

Synthesis and Activation of Vitamin D

25-hydroxyvitamin D circulates bound to vitamin D–binding protein. This protein can be filtered at the glomerulus and enter the proximal tubular facilitating the delivery of the precursor, 25-hydroxyvitamin D, to the 1α-hydroxylase. Renal 1α-hydroxylase (under regulation by PTH) is the enzyme responsible for the second step in the activation of the prehormone, the hydroxylation at C-1, resulting in the hormonally active vitamin D [1,25(OH)2D], also known as calcitriol. Calcitriol is released into the circulation (20–60 pg/mL), where it functions as an endocrine hormone, regulating cellular processes in a host of target tissues. This second hydroxylation step, the production of 1,25(OH)2D by 1α-hydroxylase in the kidney, is a tightly regulated process and is a central factor in the feedback regulation of calcium homeostasis. The production of the active form of vitamin D is under negative feedback regulation by plasma Ca2+ levels. An increase in plasma Ca2+ levels inhibits the hydroxylation at C-1 and favors hydroxylation at C-24, leading to the synthesis of an inactive metabolite of vitamin D [24,25(OH)2D].

In summary, PTH stimulates the activity of 1α-hydroxylase, favoring an increase in the synthesis of the active form of vitamin D. Vitamin D, as well as high Ca2+ levels, suppress the activity of 1α-hydroxylase, decreasing its own synthesis and favoring the synthesis of 24,25(OH2)D, the less active form of the hormone (Figure 5–9).

Cellular Effects of Vitamin D

Vitamin D is able to generate biologic effects both by genomic mechanisms (changes in gene transcription) and rapid nongenomic mechanisms. The genomic effects are dependent on the interaction of 1,25(OH)2D with a cytosolic-nuclear receptor protein, followed by interaction of the steroid-receptor complex in the nucleus with selective regions of the promoters of genes that are either activated or repressed. The stimulation of rapid responses by 1,25(OH)2D may result from interaction of the vitamin with a cell membrane receptor for 1,25(OH)2D that activates a variety of signal transduction systems, including protein kinase C, phospholipase C, and adenylate cyclase, and modulates ion (ie, Ca2+ or Cl–) channels.

Abnormal Vitamin D Levels

The US recommended daily allowance of vitamin D is 200 U for adults and 400 U for children, pregnant women, and lactating women. Vitamin D belongs to the class of vitamins that are lipid soluble (ie, A, D, E, and K) and can be stored in tissues. Excess vitamin D may lead to problems such as calcinosis (calcification of soft tissues), deposition of Ca2+ and PO4 in the kidney, and increased plasma Ca2+ levels, resulting in cardiac arrhythmia.

Deficiency of vitamin D can be the result of decreased dietary intake or lack of sunlight and the resulting decreased conversion from the inactive precursor in skin to the active form of the vitamin. Vitamin D deficiency results in bone deformities (rickets) when it occurs in children and decreased bone mass (osteomalacia) in adults. Vitamin D deficiency is associated with weakness, bowing of the weight-bearing bones, dental defects, and hypocalcemia. Factors that may contribute to vitamin D deficiency include the use of sunscreen, particularly in the elderly population; lack of sunlight during November through March in certain latitudes (above 40 N and below 40 S); and use of clothing that covers most of the skin. Less frequently, people may have a mutation in 1α-hydroxylase, the enzyme that catalyzes the second and final step in vitamin D activation, or a resistance to vitamin D action in the tissues caused by mutations in the vitamin D receptor.

Role of Calcitonin in Calcium Homeostasis

A third hormone involved in calcium homeostasis, although to a lesser extent than PTH and vitamin D, is calcitonin. Calcitonin is a 32–amino acid peptide hormone derived from procalcitonin, produced by cells of neural crest origin (parafollicular or C cells) in the thyroid gland. Calcitonin belongs to a family of peptides including amylin, calcitonin gene-related peptides (CGRPs) and adrenomedullin. These are distributed in various peripheral tissues as well as in the central nervous system and induce multiple biologic effects including potent vasodilatation (CGRP and adrenomedullin), reduction in nutrient intake (amylin), and decreased bone resorption (calcitonin).

The release of calcitonin is regulated by plasma calcium levels through a Ca2+ receptor on the parafollicular cells. Elevations in plasma Ca2+ higher than 9 mg/dL stimulate the release of calcitonin. Calcitonin has a half-life of approximately 5 minutes and is metabolized and cleared by the kidney and the liver. The release of calcitonin is also stimulated by gastrin, a gastrointestinal hormone.

Cellular Effects of Calcitonin

The main physiologic function of calcitonin is to decrease plasma Ca2+ and phosphate concentrations, mainly by decreasing bone resorption. The 2 target organs for calcitonin’s physiologic effects are bone and kidney. The overall effect of calcitonin in bone is to inhibit bone resorption, predominantly by inhibition of osteoclast motility, differentiation, and ruffled border formation. Calcitonin inhibits osteoclast secretory activity (particularly of tartrate-resistant acid phosphatase), alters Na+-K+-ATPase activity, carbonic anhydrase localization, and inhibits H+-ATPase activity, reducing osteoclast acid secretion. In the kidney, calcitonin increases urinary Ca2+ excretion by inhibition of renal tubular calcium reabsorption. The mechanism involved is through opening of low affinity Ca2+ channels in the luminal membrane and the stimulation of the Na+/Ca2+ exchanger in the basolateral membrane, both actions depending on the activation of adenylate cyclase. In hypercalcemic patients with metastatic bone disease, the administration of calcitonin induces a rapid decrease in plasma calcium primarily through inhibition of renal tubular reabsorption.

Calcitonin Receptors

The cellular effects of calcitonin are mediated through G protein (Gs, Gq, or Gi)–coupled receptors from the same receptor family as the PTH, PTHrP, calcitonin, adrenomedullin, secretin receptor superfamily. Several calcitonin receptor subtypes have been identified, and they all bind calcitonin with high affinity. Calcitonin binding to its receptor stimulates adenylate cyclase, increasing the formation of cAMP and activation of protein kinase A and phospholipase C, resulting in the release of Ca2+ from intracellular stores and influx of extracellular Ca2+.

Calcitonin and Disease

Calcitonin does not appear to be critical for the regulation of calcium homeostasis; in fact, total removal of the thyroid does not produce major alterations in Ca2+ homeostasis. In addition, no significant clinical findings have been associated with calcitonin excess or deficiency.

However, calcitonin has been used therapeutically for the prevention of bone loss and for the short-term treatment of hypercalcemia of malignancy. Osteoporosis is a systemic skeletal disease characterized by low bone mass and deterioration of bone tissue, resulting in bone fragility and susceptibility to fracture (discussed in Chapter 10). The ability of calcitonin to inhibit osteoclast-mediated bone resorption has made it a useful agent for the treatment of osteoporosis; it also relieves pain in osteoporotic patients with vertebral crush fractures. Calcitonin is also used in the treatment of Paget disease, which is characterized by an abnormality in bone remodeling, with increased bone resorption and hypercalcemia.

Additional Regulators of Ca2+ and Bone Metabolism

Although PTH and vitamin D play central roles in the regulation of bone metabolism, the contribution of other hormones cannot be ignored (Table 5–2). Sex steroids (androgens and estrogens) have been shown to increase 1α-hydroxylase activity, decrease bone resorption, and increase osteoprotegerin synthesis. Estrogen stimulates the proliferation of osteoblasts and the expression of type I collagen and alkaline phosphatase; influences the expression of receptors for vitamin D, growth hormone, and progesterone; and modulates responsiveness of bone to PTH. Estrogen decreases the number and activity of osteoclasts, as well as the synthesis of cytokines affecting bone resorption.

Growth hormone and insulin-like growth factor-1 (IGF-1) both exert effects on bone metabolism. Growth hormone stimulates the proliferation and differentiation of osteoblasts and bone protein synthesis and growth. IGF-1 produced by the liver and locally by osteoblasts, stimulate bone formation by increasing the proliferation of osteoblast precursors and by enhancing the synthesis and inhibiting the degradation of type I collagen. Normal thyroid function is required for physiologic bone remodeling. However, excess thyroid hormone levels result in increased bone resorption. Prolactin increases Ca2+ reabsorption and 1α-hydroxylase activity, indirectly modulating bone metabolism. Glucocorticoids play an overall catabolic role in bone metabolism by increasing bone resorption and decreasing bone synthesis, resulting in an increase in the risk of fractures. The mechanisms by which glucocorticoids exert their effects are not fully understood, but inhibition of osteoprotegerin may help stimulate osteoclastic bone resorption. The cytokines tumor necrosis factor, interleukin 1, and interleukin 6 increase the proliferation and differentiation of osteoclast precursors and their osteoclastic activity and are therefore potent stimulators of bone resorption in vitro and in vivo. The overall interaction of these various factors during health and disease plays an important role in maintaining bone mass. Their specific contributions may vary depending on the disease and on the prevailing hormone and cytokine levels in bone.

Hormonal Regulation of Bone Metabolism

Bone remodeling results from the interactions of multiple elements, including osteoblasts, osteoclasts, hormones, growth factors, and cytokines, the result being a dynamic maintenance of the bone architecture and systemic preservation of calcium homeostasis. Quiescent bone is covered by flat bone-lining cells. During bone resorption, osteoclasts are recruited and activated to remove both organic matrix and mineral content of bone to produce a pit. During bone formation, osteoblasts deposit osteoid in the pit, which is then mineralized under osteoblastic control. Hormones can influence bone remodelling at any stage throughout the remodelling cycle through direct effects on either osteoblasts or osteoclasts to alter either bone resorption or bone formation. It is important to remember that, in vivo, normal bone structure is maintained by complex interactions between osteoblasts and osteoclasts.

In early life, a careful balance exists between bone formation by osteoblasts and bone resorption by osteoclasts. With aging, the process of coupled bone formation-resorption is affected by the reductions in osteoblast differentiation, activity, and life span, which are further potentiated in the perimenopausal years by hormone deprivation (estrogen, testosterone, and adrenal-derived androgens) and an increase in osteoclast activity.

Decreased calcium intake below obligatory calcium loss (through the urine, feces, and skin) mobilizes calcium from the skeleton to maintain the ionized calcium concentration in the ECF, resulting in bone destruction. Vitamin D deficiency lowers the concentration of ionized calcium in the ECF (from loss of the calcemic action of vitamin D on bone), resulting in stimulation of PTH release (secondary hyperparathyroidism), increased phosphate excretion leading to hypophosphatemia, and failure to mineralize new bone as it is being formed. Simple calcium deficiency is associated with compensatory increases in PTH and calcitriol, which together mobilize calcium from bone, potentially decreasing bone mass. True vitamin D deficiency, however, reduces the mineral content of the bony tissue itself and leads to abnormal bone composition. However, these 2 nutritional deficiencies cannot be completely separated because calcium malabsorption is the first manifestation of vitamin D deficiency.

Childhood and Puberty

Bone mass increases throughout childhood and adolescence. In girls, the rate of increase in bone mass decreases rapidly after menarche, whereas in boys, gains in bone mass persist up to 17 years of age and are closely linked to pubertal stage and androgen status. By age 17–23 years, the majority of peak bone mass has already been achieved in both sexes. Skeletal growth is achieved primarily through bone modeling and only partially through bone remodeling. These mechanisms involve interaction between osteoblasts and osteoclasts, which work cooperatively under the influence of the mechanical strain placed on bone by skeletal muscle force such as that exerted during exercise. The mechanical loading or strain oscillates within a given range in response to physical activity, leading to bone maintenance without loss or gain. Decreased mechanical strain (such as that associated with prolonged bed rest or immobilization) leads to bone loss, whereas increased mechanical strain (weight-bearing exercise) stimulates osteoblastic activity and bone formation. The loading on the bone cells is exerted primarily by muscles and to a lesser extent by body weight. Muscle force or tension applied on long bones increases the thickness of cortical bone through continuous subperiosteal accretion. This relationship between muscle tension exerted on bones and bone formation is positively affected during exercise.

Peak bone mass is attained in the third decade of life and is maintained until the fifth decade, when age-related bone loss begins both in men and women. Sex steroids play an important role in bone growth and the attainment of peak bone mass. They are also responsible for the sexual dimorphism of the skeleton, which emerges during adolescence and is characterized by larger bone size in males (even when corrected for body height and weight), with both a larger diameter and a greater cortical thickness in the long bones.

Pregnancy and Lactation

The uptake and release of calcium from the skeleton are increased during pregnancy, and the rate of calcium mobilization continues to be increased during the early months of lactation, returning to prepregnancy rates during or after weaning. Intestinal calcium absorption and bone mobilization are higher during pregnancy than before conception or after delivery. Urinary calcium excretion is increased during pregnancy, and may be a reflection of the increased glomerular filtration rate, exceeding calcium reabsorption capacity during that period. The increases are evident in early to midpregnancy and precede the increased demand for calcium by the fetus for skeletal growth. The alterations in calcium and bone metabolism during pregnancy are accompanied by increases in vitamin D, but without significant alterations in either intact PTH or calcitonin concentrations. The increase in intestinal calcium absorption is associated with a doubling of 1,25-dihydroxyvitamin D levels and increased intestinal expression of the vitamin D-dependent calcium-binding protein calbindin. Changes in maternal bone mineral content during this period may influence bone mineral status in the long term. After delivery, calcium absorption and urinary calcium excretion return to prepregnancy rates. However, lactating mothers have decreased urinary calcium output and higher bone turnover than at the end of pregnancy. During this period, approximately 5 mmol/d (200 mg/d) of calcium is provided to the infant through breast milk, and this can exceed 10 mmol/d (400 mg/d) in some women. Thus, requirements for calcium are significantly increased during pregnancy and lactation.

Menopause

The acute loss of bone that accompanies menopause involves most of the skeleton but particularly affects the trabecular component. The associated biochemical changes include increases in the complexed fraction of plasma calcium (bicarbonate), increases in plasma alkaline phosphatase and urinary hydroxyproline (representing increased bone resorption followed by a compensatory increase in bone formation), increased obligatory calcium loss in the urine, and a small but significant decline in calcium absorption (Table 5–3). These changes are ameliorated by hormone treatment, calcium supplementation, thiazide administration (which reduces calcium excretion), and restriction of salt intake, which reduces obligatory calcium loss. In some (50%) cases of osteoporosis, calcium absorption is low, and high bone resorption can be suppressed by treatment with vitamin D which in turn leads to improvement in calcium absorption. In males, bone loss begins at approximately the age of 50 years, but it is not associated with an increase in bone resorption markers. Instead, bone loss in men is linked to an age-related decline in gonadal function and is caused by a decrease in bone formation, not so much as an increase in bone resorption.

Estrogen deficiency is a major pathogenic factor in the bone loss associated with menopause and the subsequent development of postmenopausal osteoporosis. Estrogen replacement at or after menopause, whether natural or induced, prevents menopausal bone loss and usually results in an increase in bone mineral density (BMD) during the first 12–18 months of treatment. Estrogen regulates osteoclast activity through effects on osteoclast number, resorptive activity, and life span of the cell. The process of bone loss is progressive, starting at approximately the age of 50 in men and at menopause in women, and loss proceeds at an average rate of 1% per year to the end of life. Bone loss is faster in women than in men and affects some bones more than others; the consequences include decreased BMD and increased risk of fractures.

Bone Density

Bone density determines the degree of osteoporosis and the fracture risk. The main determinants of peak bone density are genetics, calcium intake, and exercise. The most common test for measuring bone density is dual-energy x-ray absorptiometry (DEXA) scanning. Additional approaches include computed tomography, radiologic techniques (morphometry or densitometry), or bone biopsy. DEXA uses x-rays to measure bone density and provides 2 measures of how dense bone is: the T score and the Z score. The T score compares the person’s bone density with the average bone density of 25- to 30-year-olds of the same sex. This age group is used because it is when bone density is at its peak. A T score of 0 means that bone density is the same as the average bone density of 25- to 30-year-olds. A score above 0 (a positive score) means that bones are more dense than the average. A score below 0 (a negative score) means that bones are less dense than the average. The Z score compares a person’s bone density with that of people of the same age, sex, and weight, and is less valuable in making predictions of risk of fracture or in making decisions about treatment.

Prevention of Osteoporosis

Understanding of the hormonal and nutritional regulation of calcium balance has led to the implementation of several measures to reduce bone loss. The principal current approaches are the following.

Estrogen Replacement Therapy

Estrogen decreases bone loss in postmenopausal women by inhibiting bone resorption, resulting in a 5%–10% increase in BMD over 1–3 years. Estrogen treatment is approved for prevention of osteoporosis. Calcium supplements enhance the effect of estrogen on BMD.

Bisphosphonates

Bisphosphonates have a strong affinity for bone apatite and are potent inhibitors of bone resorption. Bisphosphonates reduce the recruitment and activity of osteoclasts and increase their apoptosis.

Calcitonin

Calcitonin reduces bone resorption by direct inhibition of osteoclast activity. Calcitonin is less effective in prevention of cortical bone loss than cancellous bone loss in postmenopausal women. Calcitonin is approved for treatment of osteoporosis in women who have been postmenopausal for 5 or more years.

Parathyroid Hormone

Human recombinant PTH is approved for the treatment of osteoporosis in postmenopausal women and men who are at high risk for fracture. Intermittent PTH administration stimulates new bone formation at the periosteal (outer) and endosteal (inner) bone surfaces and thickening the cortices and existing trabeculae of the skeleton.

Selective Estrogen Receptor Modulators

Selective estrogen receptor modulators (SERMs) are compounds that exert estrogenic effects in specific tissues and antiestrogenic effects in others. Raloxifene, a SERM competitively inhibits the action of estrogen in the breast and the endometrium and acts as an estrogen agonist on bone and lipid metabolism. In early postmenopausal women, raloxifene prevents postmenopausal bone loss at all skeletal sites, reduces markers of bone turnover to premenopausal concentrations, and reduces the serum cholesterol concentration and its low-density lipoprotein fraction without stimulating proliferation in the endometrium. Because raloxifene does not have agonistic effects on the endometrium, unwanted vaginal bleeding and increased risk of endometrial cancer are avoided. Thus, it exerts the beneficial effects of estrogen in the skeleton and cardiovascular system without its adverse effects in the breast and endometrium.

Vitamin D Analogs

Vitamin D analogs induce a small increase in BMD that seems to be limited to the spine.

Exercise

Physical activity early in life contributes to high peak bone mass. Various activities, including walking, weight training, and high-impact exercises, induce a small (1%–2%) increase in BMD at some, but not all, skeletal sites. These effects disappear if the exercise program is stopped. Load-bearing exercise is more effective for increasing bone mass than are other types of exercise. Some of the benefits of exercise may be caused by increases in muscle mass and strength, plus a reduction in the risk of falls by approximately 25% in frail elderly individuals.

Primary Hyperparathyroidism

Primary hyperparathyroidism is the result of excess PTH production caused by parathyroid gland hyperplasia, adenoma, or carcinoma. The clinical manifestations include elevated intact PTH levels; increased plasma calcium levels (hypercalcemia); increased urinary calcium excretion (hypercalciuria), which may lead to increased formation of kidney stones (urolithiasis); and decreased plasma phosphate levels. The elevation of PTH results in increased bone resorption and further increases in extracellular calcium concentrations.

Secondary Hyperparathyroidism

Secondary hyperparathyroidism, is the result of alterations outside of the parathyroid gland. Most frequently, it is a complication that occurs in patients with chronic renal failure. In early renal failure, a reduction in plasma vitamin D and moderate decreases in ionized Ca2+ contribute to greater synthesis and secretion of PTH. As renal disease progresses, parathyroid expression of vitamin D and Ca2+ receptors is reduced, making the parathyroid gland very resistant to both the vitamin D and Ca2+ negative feedback regulation of PTH release. Thus, for any increase in plasma Ca2+, the inhibition of PTH secretion is less efficient. As a result, for any particular plasma Ca2+ concentration, secretion of PTH is enhanced, resulting in a shift in the Ca2+-PTH set point toward secondary hyperparathyroidism. Hyperphosphatemia (higher than 5 mg/dL in adults), independent of Ca2+ and calcitriol levels, further enhances uremia-induced parathyroid gland hyperplasia and PTH synthesis and secretion, the latter by post-transcriptional mechanisms as discussed in Chapter 10.

Hypoparathyroidism

Hypoparathyroidism or low PTH levels may result from surgical removal of the parathyroid glands or can be associated with other endocrine disorders and neoplasias. Because of the important role of PTH in the acute regulation of plasma Ca2+ levels, an early manifestation of surgical removal of the parathyroid glands is hypocalcemic tetany. The classic clinical sign is known as the Chvostek sign, which is twitching or contraction of the facial muscles in response to tapping the facial nerve at a point anterior to the ear and above the zygomatic bone.

Pseudohypoparathyroidism

Pseudohypoparathyroidism or “not real” hypoparathyroidism is not caused by decreased PTH levels, but by an abnormal response to PTH because of a congenital defect in the G protein associated with the PTHR1. Pseudohypoparathyroidism type Ia is characterized by a generalized hormone resistance to PTH, thyroid-stimulating hormone, luteinizing hormone, and follicle-stimulating hormone and is associated with abnormal physical features including short stature and skeletal anomalies. Pseudohypoparathyroidism type Ib is characterized by renal resistance to PTH and normal appearance. In these patients, the PTHR1 has decreased Gαs activity, and this can be tested by measuring the increase in urinary cAMP in response to PTH administration (which should be low in patients with defective Gαs activity). Patients present with low plasma Ca2+ (caused by inability of PTH to increase calcium reabsorption), high phosphate levels (caused by inability to excrete phosphate), and elevated PTH levels (parathyroid gland’s attempt to respond to the low calcium and elevated phosphate levels).

Clinical Evaluation of Parathyroid Hormone Abnormalities

Patients’ clinical manifestations can be combined with indices of hormone levels (PTH and vitamin D), the metabolites that the hormones regulate (calcium and Pi), and the target tissue effects of excess hormone production (bone densitometry). In addition, magnesium levels, when extremely low can prevent PTH release in response to hypocalcemia, so their determination is also important for overall evaluation. As seen in the brief overview of some of these measures (see Table 5–3 and Appendix), the laboratory values indicate the physiologic regulation of PTH, vitamin D, phosphate, and calcium levels. Plasma PTH levels are determined using radioimmunoassay or more specific and sensitive radioimmunometric assays. Because PTH loses its activity once it is cleaved, only the intact PTH measurement is a reliable indicator of hormone status. Vitamin D levels can be measured, although this test is expensive and time consuming; therefore, the concentrations of the precursor 25-hydroxyvitamin D are used more frequently. Bone density studies using DEXA are used to determine the extent of bone loss. Measurements are usually obtained in the lumbar spine or femur. When there is a possibility of increased bone turnover, as would occur with increased PTHrP production in some malignancies, bone scans are used to assess the rate of incorporation of radiolabeled phosphate. The greater the rate of bone formation, the greater the deposition of labeled phosphate in bone, reflecting an increase in bone turnover. Generalized increases are observed in hyperparathyroidism, whereas focal increases are observed in malignant diseases.

• PTH release is controlled by circulating Ca2+ levels and is under negative feedback regulation by Ca2+ and vitamin D.
• The main physiologic effects of PTH are mediated by the PTHR1 expressed in bone and kidney.
• PTHR1 binds PTH and PTHrP, a peptide responsible for the pathophysiologic elevation of Ca2+ in some malignancies.
• In the kidney, PTH increases renal Ca2+ reabsorption, increases the activity of 1α-hydroxylase (which mediates the final activation step in the synthesis of vitamin D), and decreases Pi reabsorption.
• In bone, PTH increases osteoclast-mediated bone resorption indirectly through stimulation of osteoblast activity.
• Calcitonin decreases bone resorption and lowers plasma Ca2+ levels.
• Synthesis of active vitamin D involves hydroxylation in the liver (C-25) and kidney (C-1).
• Vitamin D increases intestinal Ca2+ absorption, facilitates renal Ca2+ reabsorption, and bone resorption.