3168 PART 12 Endocrinology and Metabolism

of vitamin D deficiency in obese individuals is poorly understood. In

addition to intestinal diseases, accelerated inactivation of vitamin D

metabolites can be seen with drugs that induce hepatic cytochrome

P450 mixed-function oxidases such as barbiturates, phenytoin, and

rifampin. Gain-of-function mutations in CYP3A4 accelerate the oxidation and inactivation of vitamin D metabolites, thus resulting in

decreased serum levels of 25OHD and 1,25(OH)2

D. This form of

rickets is autosomal recessive and presents during early childhood and

can be treated with high doses of calcitriol or vitamin D. Impaired

25-hydroxylation, associated with severe liver disease or isoniazid, is

an uncommon cause of vitamin D deficiency. A mutation in the gene

responsible for 25-hydroxylation has been identified in a few kindreds.

Increased circulating FGF23 levels impair 1α-hydroxylation, preventing the production of 1,25(OH)2

D. High levels of FGF23 are seen in

those with genetic disorders associated with hypophosphatemic rickets, the most common of which is X-linked hypophosphatemia, and

are prevalent in populations with profound renal dysfunction. Thus,

therapeutic interventions should be considered in patients whose

creatinine clearance is <0.5 mL/s (30 mL/min). Mutations in the renal

1α-hydroxylase are the basis for the genetic disorder pseudovitamin

D–deficiency rickets (also called vitamin D–dependent rickets type I).

This autosomal recessive disorder presents with the syndrome of vitamin D deficiency in the first year of life. Patients present with growth

retardation, rickets, and hypocalcemic seizures. Serum 1,25(OH)2

D

levels are low despite normal 25(OH)D levels and elevated PTH

levels. Treatment with vitamin D metabolites that do not require 1αhydroxylation for activity results in disease remission, although lifelong

therapy is required. A second autosomal recessive disorder, hereditary

vitamin D–resistant rickets (also called vitamin D-dependent rickets

type II), a consequence of vitamin D receptor mutations, is a greater

therapeutic challenge. These patients present in a similar fashion during the first year of life, but alopecia often accompanies the disorder,

demonstrating a functional role of the VDR in the keratinocyte stem

cell population required for hair follicle regeneration. Serum levels

of 1,25(OH)2

D are dramatically elevated in these individuals both

because of increased production due to stimulation of 1α-hydroxylase activity as a consequence of secondary hyperparathyroidism and

because of impaired inactivation since induction of the 24-hydroxylase

by 1,25(OH)2

D requires an intact VDR. Since the receptor mutation

results in hormone resistance, daily calcium and phosphate infusions may be required to bypass the defect in intestinal mineral ion

absorption.

Regardless of the cause, the clinical manifestations of vitamin D

deficiency are largely a consequence of impaired intestinal calcium

absorption. Mild to moderate vitamin D deficiency is asymptomatic,

whereas long-standing vitamin D deficiency results in hypocalcemia

accompanied by secondary hyperparathyroidism, impaired mineralization of the skeleton (osteopenia on x-ray or decreased bone mineral

density), and proximal myopathy. Vitamin D deficiency also has been

shown to be associated with an increase in overall mortality, including

cardiovascular causes. In the absence of an intercurrent illness, the

hypocalcemia associated with long-standing vitamin D deficiency

rarely presents with acute symptoms of hypocalcemia such as numbness, tingling, and seizures. However, the concurrent development of

hypomagnesemia, which impairs parathyroid function, or the administration of potent bisphosphonates, which impair bone resorption,

can lead to acute symptomatic hypocalcemia in vitamin D–deficient

individuals.

Rickets and Osteomalacia In children, before epiphyseal fusion,

vitamin D deficiency results in growth retardation associated with

an expansion of the growth plate known as rickets. Three layers of

chondrocytes are present in the normal growth plate: the reserve zone,

the proliferating zone, and the hypertrophic zone. Rickets associated

with impaired vitamin D action is characterized by expansion of the

hypertrophic chondrocyte layer. The expansion of the growth plate is a

consequence of impaired apoptosis of the late hypertrophic chondrocytes, an event that precedes replacement of these cells by osteoblasts

during endochondral bone formation. Investigations in murine models

demonstrate that hypophosphatemia, which in vitamin D deficiency

is a consequence of secondary hyperparathyroidism, is a key etiologic

factor in the development of the rachitic growth plate. Impaired actions

specific to vitamin D also contribute to the expansion of the hypertrophic layer in the rachitic growth plate.

The hypocalcemia and hypophosphatemia that accompany vitamin

D deficiency result in impaired mineralization of bone matrix proteins,

a condition known as osteomalacia. Osteomalacia is also a feature

of long-standing hypophosphatemia, which may result from renal

phosphate wasting, or chronic use of etidronate or phosphate-binding

antacids. This hypomineralized matrix is biomechanically inferior to

normal bone; as a result, patients with osteomalacia are prone to bowing of weight-bearing extremities and skeletal fractures. Vitamin D and

calcium supplementation have been shown to decrease the incidence

of hip fracture among ambulatory nursing home residents in France,

suggesting that undermineralization of bone contributes significantly

to morbidity in the elderly. Proximal myopathy is a striking feature of

severe vitamin D deficiency both in children and in adults. Rapid resolution of the myopathy is observed upon vitamin D treatment.

Although vitamin D deficiency is the most common cause of rickets

and osteomalacia, many disorders lead to inadequate mineralization of

the growth plate and bone. Calcium deficiency without vitamin D deficiency, the disorders of vitamin D metabolism previously discussed,

and hypophosphatemia can all lead to inefficient mineralization. Even

in the presence of normal calcium and phosphate levels, chronic acidosis and drugs such as bisphosphonates can lead to osteomalacia. The

inorganic calcium/phosphate mineral phase of bone cannot form at

low pH. Bisphosphonates bind to and prevent hydroxyapatite crystal

growth. Since alkaline phosphatase is necessary for normal mineral

deposition, probably because the enzyme can hydrolyze inhibitors of

mineralization such as inorganic pyrophosphate, genetic inactivation

of the alkaline phosphatase gene (hereditary hypophosphatasia) also

can lead to osteomalacia in the setting of normal calcium and phosphate levels.

Diagnosis of Vitamin D Deficiency, Rickets, and Osteomalacia

The most specific screening test for vitamin D deficiency in otherwise

healthy individuals is a serum 25(OH)D level. Although the normal

ranges vary, levels of 25(OH)D <37 nmol/L (<15 ng/mL) are associated with increasing PTH levels and lower bone density. The National

Academy of Medicine has defined vitamin D sufficiency as a vitamin D

level >50 nmol/L (>20 ng/mL), although higher levels may be required

to optimize intestinal calcium absorption in the elderly and those with

underlying disease states, including obesity. Vitamin D deficiency leads

to impaired intestinal absorption of calcium, resulting in decreased

serum total and ionized calcium values. This hypocalcemia results in

secondary hyperparathyroidism, a homeostatic response that initially

maintains serum calcium levels at the expense of the skeleton. Due

to the PTH-induced increase in bone turnover, alkaline phosphatase

levels are often increased. In addition to increasing bone resorption,

TABLE 409-6 Causes of Impaired Vitamin D Action

Vitamin D deficiency Impaired 1α-hydroxylation

Impaired cutaneous production Hypoparathyroidism

Dietary absence Ketoconazole

 Malabsorption (short gut syndrome,

gastric bypass)

1α-hydroxylase mutation

FGF23 excess

Oncogenic osteomalacia

Hypophosphatemic rickets

Fibrous dysplasia

Chronic kidney disease

Target organ resistance

Vitamin D receptor mutation

Phenytoin

Other

Obesity

Accelerated loss of vitamin D

 Increased metabolism (barbiturates,

phenytoin, rifampin)

Impaired enterohepatic circulation

Nephrotic syndrome

CYP3A4 mutation

Impaired 25-hydroxylation

Liver disease, isoniazid

25-Hydroxylase mutation

3169 Disorders of the Parathyroid Gland and Calcium Homeostasis CHAPTER 410

PTH decreases urinary calcium excretion while promoting phosphaturia. This results in hypophosphatemia, which exacerbates the

mineralization defect in the skeleton. With prolonged vitamin D deficiency resulting in osteomalacia, calcium stores in the skeleton become

relatively inaccessible, since osteoclasts cannot resorb unmineralized

osteoid, and frank hypocalcemia ensues. Since PTH is a major stimulus

for the renal 25(OH)D 1α-hydroxylase, there is increased synthesis of

the active hormone, 1,25(OH)2

D. Paradoxically, levels of this hormone

are often normal in severe vitamin D deficiency. Therefore, measurements of 1,25(OH)2

D are not accurate reflections of vitamin D stores

and should not be used to diagnose vitamin D deficiency in patients

with normal renal function.

Radiologic features of vitamin D deficiency in children include a

widened, expanded growth plate that is characteristic of rickets. These

findings not only are apparent in the long bones but also are present

at the costochondral junction, where the expansion of the growth

plate leads to swellings known as the “rachitic rosary.” Impairment of

intramembranous bone mineralization leads to delayed fusion of the

calvarial sutures and a decrease in the radiopacity of cortical bone in

the long bones. If vitamin D deficiency occurs after epiphyseal fusion,

the main radiologic finding is a decrease in cortical thickness and

relative radiolucency of the skeleton. A specific radiologic feature of

osteomalacia, whether associated with phosphate wasting or vitamin D

deficiency, is pseudofractures, or Looser’s zones. These are radiolucent

lines that occur where large arteries are in contact with the underlying

skeletal elements; it is thought that the arterial pulsations lead to the

radiolucencies. As a result, these pseudofractures are usually a few millimeters wide, are several centimeters long, and are seen particularly in

the scapula, the pelvis, and the femoral neck.

TREATMENT

Vitamin D Deficiency

Based on the National Academy of Medicine 2010 report, the recommended daily intake of vitamin D is 600 IU from 1 to 70 years of

age, and 800 IU for those >70. Based on the observation that 800 IU

of vitamin D, with calcium supplementation, decreases the risk of

hip fractures in elderly women, this higher dose is thought to be

an appropriate daily intake for prevention of vitamin D deficiency

in adults. The Vitamin D and Omega-3 Trial (VITAL) revealed

that supplementation of vitamin D in people >50 years of age with

normal vitamin D levels, at doses above the recommended daily

intake, does not further improve bone mineral density or skeletal

microarchitecture and does not prevent falls. The safety margin

for vitamin D is large, and vitamin D toxicity usually is observed

only in patients taking doses in the range of 40,000 IU daily. Treatment of vitamin D deficiency should be directed at the underlying

disorder, if possible, and also should be tailored to the severity of

the condition. Vitamin D should always be repleted in conjunction

with calcium supplementation since most of the consequences of

vitamin D deficiency are a result of impaired mineral ion homeostasis. In patients in whom 1α-hydroxylation is impaired, metabolites

that do not require this activation step are the treatment of choice.

They include 1,25(OH)2

D3

 (calcitriol [Rocaltrol], 0.25–0.5 μg/d)

and 1α-hydroxyvitamin D2

 (doxercalciferol [Hectorol], 2.5–5 μg/d).

Outside the United States, 1α-hydroxyvitamin D3, (alfacalcidol

[One-Alpha] (0.25–1.0 μg/d) is also used. If the pathway required

for activation of vitamin D is intact, severe vitamin D deficiency

can be treated with pharmacologic repletion initially (50,000 IU

weekly for 3–12 weeks), followed by maintenance therapy (800

IU daily). Pharmacologic doses may be required for maintenance

therapy in patients who are taking medications such as barbiturates or phenytoin that accelerate metabolism of or cause resistance to 1,25(OH)2

D. Polymorphisms in the 25-hydroxylase and

the 24-hydroxylase genes can also lead to different responses to

the normal recommended daily intake of vitamin D. The hepatic

enzyme cytochrome P450 3A4 (CYP3A4) is a strong inducer of

the catabolism of vitamin D metabolites. Polymorphisms of the

CYP3A4 gene and certain drugs, such as phenytoin and rifampin,

lead to strong induction of this enzyme; thus, those affected may

also require higher doses of vitamin D supplementation. Calcium

supplementation should include 1.5–2 g/d of elemental calcium.

Normocalcemia is usually observed within 1 week of the institution

of therapy, although increases in PTH and alkaline phosphatase

levels may persist for 3–6 months. The most efficacious methods

to monitor treatment and resolution of vitamin D deficiency are

serum and urinary calcium measurements. In patients who are

vitamin D replete and are taking adequate calcium supplementation, the 24-h urinary calcium excretion should be in the range of

100–250 mg/24 h. Lower levels suggest problems with adherence

to the treatment regimen or with absorption of calcium or vitamin

D supplements. Levels >250 mg/24 h predispose to nephrolithiasis

and should lead to a reduction in vitamin D dosage and/or calcium

supplementation.

Acknowledgement

The authors would like to acknowledge the advice of Marie Demay (also

a former author of this chapter), Michael Mannstadt, and Marc Wein,

who helped us put this chapter together.

FURTHER READING

Amrein K et al: Vitamin D deficiency 2.0: An update on the current

status worldwide. Eur J Clin Nutr 74:1498, 2020.

Bikle D et al: Vitamin D metabolites in captivity? Should we measure

free or total 25(OH)D to assess vitamin D status? J Steroid Biochem

Mol Biol 173:105, 2017.

Carpenter TO et al: Burosumab therapy in children with X-linked

hypophosphatemia. N Engl J Med 378:1987, 2018.

Christakos S et al: Vitamin D: Metabolism, molecular mechanism of

action, and pleiotropic effects: Physiol Rev 96:365, 2016.

De Baaij JH et al: Magnesium in man: Implications for health and

disease. Physiol Rev 95:1, 2015.

Institute of Medicine (IOM): Dietary Reference Intakes for Calcium

and Vitamin D. Washington, DC, National Academies Press, 2011.

Kim JM et al: Osteoblast-osteoclast communication and bone homeostasis. Cells 10:2073, 2020.

Robling AG, Bonewald LF: The osteocyte: New insights. Ann Rev

Physiol 82:485, 2020.

Siddiqui JA, Partridge NC: Physiological bone remodeling: Systemic

regulation and growth factor involvement. Physiology 31:233, 2016.

Zofkova I: Hypercalcemia. Pathophysiological aspects. Phys Res 65:1,

2016.

PARATHYROID GLAND DISORDERS

■ INTRODUCTION

Four parathyroid glands are located posterior to the thyroid gland.

They produce parathyroid hormone (PTH), which is the primary

regulator of calcium physiology. PTH acts directly on bone, where it

induces calcium (and phosphate) release, and on the kidney, where it

enhances calcium reabsorption in the distal tubules. In the proximal

renal tubules, PTH increases excretion of phosphate and the synthesis

of 1,25-dihydroxyvitamin D (1,25[OH]2

D), a hormone that increases

410 Disorders of the

Parathyroid Gland and

Calcium Homeostasis

John T. Potts, Jr., Harald Jüppner

3170 PART 12 Endocrinology and Metabolism

gastrointestinal calcium absorption. Serum PTH levels are tightly

regulated by a negative feedback loop. Calcium, acting through the

calcium-sensing receptor, and vitamin D, acting through its nuclear

receptor, reduce PTH release and synthesis. Additional evidence

indicates that fibroblast growth factor 23 (FGF23), a phosphaturic

hormone, can suppress PTH secretion. Understanding the hormonal

pathways that regulate calcium and phosphate levels as well as bone

metabolism is essential for effective diagnosis and management of a

wide array of hyper- and hypocalcemic disorders.

Hyperparathyroidism, characterized by excess production of PTH,

is a common cause of hypercalcemia and is usually the result of

autonomously functioning adenomas or hyperplasia. Surgery for this

disorder is highly effective and has been shown to reverse some of

the deleterious effects of long-standing PTH excess on bone density.

Humoral hypercalcemia of malignancy (HHM) is also a common

cause of hypercalcemia, which is usually due to the overproduction

of parathyroid hormone–related peptide (PTHrP) by cancer cells. The

similarities in the biochemical characteristics of hyperparathyroidism

and HHM, first noted by Albright in 1941, are now known to reflect

the actions of PTH and PTHrP through the same G protein–coupled

PTH/PTHrP receptor (PTHR1). The converse, namely hypocalcemia,

can be caused by the lack of functional PTH, i.e., hypoparathyroidism,

or by reduced PTH responsiveness of the proximal renal tubules, i.e.,

pseudohypoparathyroidism (PHP).

The genetic basis of hyperparathyroidism, multiple endocrine

neoplasia (MEN) types 1 and 2, familial hypocalciuric hypercalcemia

(FHH), Jansen’s syndrome, different forms of hypoparathyroidism and

PHP, disorders of excess urinary phosphate excretion and of vitamin D

synthesis, action, and metabolism, and the molecular events associated

with parathyroid gland neoplasia has provided new insights into the

regulation of calcium and phosphate homeostasis. In addition, PTH

and possibly some of its analogues are promising therapeutic agents

for the treatment of postmenopausal or senile osteoporosis, and calcimimetic agents, which activate the calcium-sensing receptor, have

provided new approaches for PTH suppression.

PARATHYROID GLAND DISORDERS

■ PTH

Structure and Physiology PTH is an 84-amino-acid single-chain

peptide. The amino-terminal portion, PTH(1–34), is highly conserved

and is critical for the biologic actions of the molecule. Modified synthetic fragments of the amino-terminal sequence as small as PTH(1–11)

are sufficient to activate the PTH/PTHrP receptor, if provided at high

enough concentrations (see below). The carboxyl-terminal portions

of full-length PTH(1–84) also can bind to a separate binding protein/

receptor; however, its properties and biologic role(s), if any, remain

undefined.

The primary function of PTH is to maintain the extracellular fluid

(ECF) calcium concentration within a narrow normal range. The hormone acts directly on bone and kidney and indirectly on the intestine

through its effects on synthesis of 1,25(OH)2

D to increase serum calcium concentrations; in turn, PTH production is closely regulated by

the concentration of serum ionized calcium. This feedback system is

the critical homeostatic mechanism for maintenance of ECF calcium.

Any tendency toward hypocalcemia, as might be induced by calciumor vitamin D–deficient diets, is counteracted by an increased secretion

of PTH. This in turn (1) increases the rate of dissolution of bone

mineral, thereby increasing the flow of calcium (and phosphate) from

bone into blood; (2) reduces the renal clearance of calcium, returning

more of the calcium and phosphate filtered at the glomerulus into ECF;

and (3) increases the efficiency of calcium absorption in the intestine

by stimulating the production of 1,25(OH)2

D. Immediate control of

blood calcium is due to PTH effects on bone and, to a lesser extent, on

renal calcium clearance. Maintenance of steady-state calcium balance,

on the other hand, probably results from the effects of 1,25(OH)2

D

on calcium absorption (Chap. 409). The renal actions of PTH are

exerted at multiple sites and include an increase in urinary phosphate

excretion (proximal tubule), augmentation of calcium reabsorption

(distal tubule), and stimulation of the renal 25(OH)D-1α-hydroxylase.

As much as 12 mmol (500 mg) of calcium is transferred between the

ECF and bone each day (a large amount in relation to the total ECF

calcium pool), and PTH has a major effect on this transfer. The homeostatic role of the hormone can preserve calcium concentration in blood

at the cost of bone demineralization.

PTH has multiple actions on bone, some direct and some indirect.

PTH-mediated changes in bone calcium release can be seen within

minutes. The chronic effects of PTH are to increase the number of

bone cells, both osteoblasts and osteoclasts, and to increase the remodeling of bone; these effects are apparent within hours after the hormone

is given and persist for hours after PTH is withdrawn. Continuous

exposure to elevated PTH (as in hyperparathyroidism or long-term

infusions in animals) leads to increased osteoclast-mediated bone

resorption. However, the intermittent administration of small amounts

of PTH, elevating hormone levels for 1–2 h each day, leads to a net

increase of bone mass rather than bone loss. Striking increases, especially in trabecular bone in the spine and hip, have been reported with

the use of PTH. PTH(1–34) as monotherapy caused a highly significant

reduction in fracture incidence in a worldwide placebo-controlled trial.

Osteoblasts (or their stromal cell precursors), which have PTH/

PTHrP receptors, are crucial to this bone-forming effect of PTH. When

PTH activates PTH/PTHrP receptors on osteocytes, release of calcium

from the matrix surrounding these cells is enhanced; osteoclasts, which

mediate bone breakdown, lack such receptors. PTH-mediated stimulation of osteoclasts is indirect, acting in part through cytokine released

from osteoblasts to activate osteoclasts; in experimental studies of bone

resorption in vitro, osteoblasts must be present for PTH to activate

osteoclasts to resorb bone (Chap. 409).

Synthesis, Secretion, and Metabolism • SYNTHESIS Parathyroid cells have multiple methods of adapting to increased needs

for PTH production. Most rapid (within minutes) is secretion of preformed hormone in response to hypocalcemia. Second, within hours,

PTH mRNA expression is induced by sustained hypocalcemia. Finally,

protracted challenge leads within days to cellular replication to increase

parathyroid gland mass.

PTH is initially synthesized as a larger molecule (preproparathyroid

hormone, consisting of 115 amino acids). After a first cleavage step to

remove the “pre” sequence of 25 amino acid residues, a second cleavage

step removes the “pro” sequence of 6 amino acid residues before secretion of the mature peptide comprising 84 residues. Mutations in the

prepro-region of the gene can cause hypoparathyroidism by interfering

with hormone synthesis, transport, or secretion.

Transcriptional suppression of the PTH gene by calcium is nearly

maximal at physiologic calcium concentrations. Hypocalcemia

increases transcriptional activity within hours. 1,25(OH)2

D strongly

suppresses PTH gene transcription. In patients with chronic kidney disease (CKD), IV administration of supraphysiologic levels of

1,25(OH)2

D or analogues of this active metabolite can dramatically

suppress PTH overproduction and is thus used clinically to control severe secondary hyperparathyroidism. Regulation of proteolytic

destruction of preformed hormone (posttranslational regulation of

hormone production) is an important mechanism for mediating

rapid (within minutes) changes in hormone availability. High calcium

increases and low calcium inhibits the proteolytic destruction of stored

hormone.

REGULATION OF PTH SECRETION PTH secretion increases steeply

to a maximum value of about five times the basal rate of secretion

as the calcium concentration falls from normal to 1.9–2.0 mmol/L

(7.6–8.0 mg/dL; measured as total calcium). However, the ionized

fraction of blood calcium is the important determinant of hormone

secretion. Severe intracellular magnesium deficiency impairs PTH

secretion (see below).

ECF calcium controls PTH secretion by interaction with a calciumsensing receptor (CaSR), a G protein–coupled receptor (GPCR) for

which Ca2+ ions act as the primary ligand (see below). This receptor,

which also has phosphate binding sites, is a member of a distinctive

3171 Disorders of the Parathyroid Gland and Calcium Homeostasis CHAPTER 410

subgroup of the GPCR superfamily that mediates its actions through

two closely related alpha-subunits of signaling G proteins, namely

Gαq and Gα11, and is characterized by a large extracellular domain

suitable for “clamping” the small-molecule ligand. Stimulation of the

CaSR by high calcium levels suppresses PTH secretion. The CaSR is

present in parathyroid glands and the calcitonin-secreting cells of the

thyroid (C cells), as well as in multiple other sites, including brain and

kidney. Genetic evidence has revealed a key biologic role for the CaSR

in parathyroid gland responsiveness to calcium and in renal calcium

clearance. Heterozygous loss-of-function mutations in CaSR cause the

syndrome of FHH, in which the blood calcium abnormality resembles

that observed in hyperparathyroidism but with hypocalciuria; two

more recently defined variants of FHH, namely FHH2 and FHH3, are

caused either by heterozygous loss-of-function mutations in Gα11, the

alpha-subunit of one of the signaling proteins downstream of the CaSR,

or by heterozygous mutations in AP2A1. Homozygous loss-of-function

mutations in the CaSR are the cause of severe neonatal hyperparathyroidism, a disorder that is typically lethal if not treated within the first

days of life. On the other hand, heterozygous gain-of-function mutations cause a form of hypocalcemia resembling hypoparathyroidism

(see below).

METABOLISM The secreted form of PTH is indistinguishable by

immunologic criteria and by molecular size from the 84-amino-acid

peptide (PTH[1–84]) extracted from glands. However, much of the

immunoreactive material found in the circulation is smaller than the

extracted or secreted hormone. The principal circulating fragments

of immunoreactive hormone lack a portion of the critical aminoterminal sequence required for biologic activity and, hence, are biologically inactive fragments (so-called middle and carboxyl-terminal

fragments). Much of the proteolysis of hormone occurs in the liver and

kidney. Peripheral metabolism of PTH does not appear to be regulated

by physiologic states (high vs low calcium, etc.); hence, peripheral

metabolism of hormone, although responsible for rapid clearance of

secreted hormone, appears to be a high-capacity, metabolically invariant catabolic process.

The rate of clearance of the secreted 84-amino-acid peptide from

blood is more rapid than the rate of clearance of the biologically inactive fragment(s) corresponding to the middle and carboxyl-terminal

regions of PTH. Consequently, the interpretation of results obtained

with earlier PTH radioimmunoassays was influenced by the nature

of the peptide fragments detected by the anti-PTH antibodies used in

these assays.

Although the problems inherent in PTH measurements have been

largely circumvented by use of double-antibody immunometric assays,

it is now known that some of these assays detect, besides the intact

molecule, large amino-terminally truncated forms of PTH, which are

present in normal and uremic individuals in addition to PTH(1–84).

The concentration of these fragments relative to that of full-length

PTH(1–84) is higher with induced hypercalcemia than in eucalcemic

or hypocalcemic conditions and is higher in patients with impaired

renal function. PTH(7–84) has been identified as a major component of these amino-terminally truncated fragments. Some evidence

suggests that the PTH(7–84) (and probably related amino-terminally

truncated fragments) can act, through yet undefined mechanisms,

as an inhibitor of PTH action and may therefore be of clinical significance, particularly in patients with CKD. In this group of patients,

efforts to prevent secondary hyperparathyroidism by a variety of measures (vitamin D analogues, higher calcium intake, higher dialysate

calcium, phosphate-lowering strategies, and calcimimetic drugs) can

lead to oversuppression of the parathyroid glands since some aminoterminally truncated PTH fragments, such as PTH(7–84), react in

many immunometric PTH assays (now termed second-generation

assays; see below under “Diagnosis”), thus overestimating the levels of biologically active, intact PTH. Excessive parathyroid gland

suppression in CKD can lead to adynamic bone disease (see below),

which has been associated in children with further impaired growth

and increased bone fracture rates in adults and can furthermore lead

to significant hypercalcemia. The measurement of PTH with newer

third-generation immunometric assays, which use detection antibodies

directed against extreme amino-terminal PTH epitopes and thus detect

only full-length PTH(1–84), may provide some advantage to prevent

bone disease in CKD.

■ PTHRP

Structure and Physiology PTHrP is responsible for most

instances of HHM (Chap. 93), a syndrome that resembles primary

hyperparathyroidism but without elevated PTH levels. Most cell types

normally produce PTHrP, including brain, pancreas, heart, lung, mammary tissue, placenta, endothelial cells, and smooth muscle. In fetal

animals, PTHrP directs transplacental calcium transfer, and high concentrations of PTHrP are produced in mammary tissue and secreted

into milk, but the biologic significance of this hormone in breast milk

is unknown. PTHrP also plays an essential role in endochondral bone

formation and in branching morphogenesis of the breast and possibly

in uterine contraction and other biologic functions.

PTH and PTHrP, although products of different genes, exhibit considerable functional and structural homology (Fig. 410-1) and have

evolved from a shared ancestral gene. The structure of the gene encoding human PTHrP, however, is more complex than that of PTH, containing multiple additional exons, which can undergo alternate splicing

patterns during formation of the mature mRNA. Protein products

of 139, 141, and 173 amino acids are produced, and other molecular

forms may result from tissue-specific degradation at accessible internal

cleavage sites. The biologic roles of these various molecular species and

the nature of the circulating forms of PTHrP are unclear. In fact, it is

uncertain whether PTHrP circulates at any significant level in healthy

children and adults. As a paracrine factor, PTHrP may be produced,

act, and be destroyed locally within tissues. In adults, PTHrP appears

to have little influence on calcium homeostasis, except in disease states,

when large tumors, especially of the squamous cell type as well as renal

cell carcinomas, lead to massive overproduction of the hormone and

hypercalcemia.

Both PTH and PTHrP bind to and activate the PTH/PTHrP receptor. The PTH/PTHrP receptor (also known as the PTH-1 receptor,

PTHR1) belongs to a subfamily of GPCRs that includes the receptors

for calcitonin, glucagon, secretin, vasoactive intestinal peptide, and a

hPTHrP

1 30 84 139

Amino acid residues

hPTH

1 30 84

Amino acid residues

hPTH SER VAL SER GLU ILE GLN LEU MET HIS ASN LEU GLY LYS HIS LEU ASN SER MET GLU ARG VAL GLU TRP LEU ARG LYS LYS LEU GLN ASP

hPTHrp ALA – – – HIS – – LEU – ASP LYS – – SER ILE GLN ASP LEU ARG – ARG PHE PHE – HIS HIS LEU ILE ALA GLU

1 5 10 15 20 25 30

FIGURE 410-1 Schematic diagram to illustrate similarities and differences in structure of human parathyroid hormone (hPTH) and human PTH-related peptide (hPTHrP).

Close structural (and functional) homology exists between the first 30 amino acids of hPTH and hPTHrP. The PTHrP sequence may be ≥139 amino acid residues in length.

PTH is only 84 residues long; after residue 30, there is little structural homology between the two. Dashed lines in the PTHrP sequence indicate identity; underlined

residues, although different from those of PTH, still represent conservative changes (charge or polarity preserved). Ten amino acids are identical, and a total of 20 of 30 are

homologues.

3172 PART 12 Endocrinology and Metabolism

few other peptides. Although both ligands activate the PTHR1, the

two peptides induce distinct responses in the receptor, which explains

how a single receptor without isoforms can serve different biologic

roles. The extracellular regions of the receptor are involved in hormone

binding, and the intracellular domains, after hormone activation,

bind G protein subunits to transduce hormone signaling into cellular

responses through the stimulation of second messenger formation. A

second receptor that binds PTH, originally termed the PTH-2 receptor

(PTH2R), is primarily expressed in brain, pancreas, and testis. Different mammalian PTHR1s respond equivalently to PTH and PTHrP, at

least when tested with traditional assays, whereas the human PTH2R

responds efficiently only to PTH, but not to PTHrP. PTH2Rs from

other species show little or no stimulation of second-messenger formation in response to PTH or PTHrP. In fact, the endogenous ligand

of the PTH2R was shown to be a hypothalamic peptide referred to as

tubular infundibular peptide of 39 residues, TIP39, that is distantly

related to PTH and PTHrP. The PTHR1 and the PTH2R can be traced

backward in evolutionary time to fish. Furthermore, the zebrafish

genome contains, in addition to the PTHR1 and the PTH2R orthologs, a third receptor, the PTH3R, that is more closely related to the

fish PTHR1 than to the fish PTH2R. The evolutionary conservation

of structure and function suggests important biologic roles for these

receptors, even in fish, which lack discrete parathyroid glands but produce two molecules that are closely related to mammalian PTH.

Studies using the cloned PTHR1 confirm that it can be coupled to

more than one G protein and second-messenger pathway, apparently

explaining the multiplicity of pathways stimulated by PTH. Activation of protein kinases (A and C) and calcium transport channels is

associated with a variety of hormone-specific tissue responses. These

responses include inhibition of phosphate and bicarbonate transport,

stimulation of calcium transport, and activation of renal 1α-hydroxylase in the kidney. The responses in bone include effects on collagen

synthesis, alkaline phosphatase, ornithine decarboxylase, citrate decarboxylase, and glucose-6-phosphate dehydrogenase activities; phospholipid synthesis; and calcium and phosphate transport. Ultimately,

these biochemical events lead to an integrated hormonal response in

bone turnover and calcium homeostasis. PTH also activates Na+/Ca2+

exchangers at renal distal tubular sites and stimulates translocation of

preformed calcium transport channels, moving them from the interior

to the apical surface to increase tubular uptake of calcium. PTHdependent stimulation of phosphate excretion involves reduced expression of two sodium-dependent phosphate co-transporters, NPT2a

and NPT2c, at the apical membrane, thereby reducing phosphate

reabsorption in the proximal renal tubules. Similar mechanisms may

be involved in other renal tubular transporters that are influenced by

PTH. Recent studies reaffirm the critical linkage of blood phosphate

lowering to net calcium entry into blood by PTH action and emphasize

the participation of bone cells other than osteoclasts in the rapid calcium-elevating actions of PTH.

PTHrP exerts important developmental influences on fetal bone

development and in adult physiology. Homozygous ablation of the

gene encoding PTHrP (or disruption of the PTHR1 gene) in mice

causes a lethal phenotype in which animals are born with pronounced

acceleration of chondrocyte maturation that resembles a lethal form of

chondrodysplasia in humans that is caused by homozygous or compound heterozygous, inactivating PTHR1 mutations (Fig. 410-2). Heterozygous inactivating PTHR1 mutations in humans furthermore can

be a cause of delayed tooth eruption, while heterozygous inactivating

PTHrP mutations lead to premature growth plate closure and reduced

adult heights.

■ CALCITONIN

(See also Chap. 388) Calcitonin is a peptide hormone with hypocalcemic properties that in several mammalian species acts as an indirect

antagonist to the calcemic actions of PTH. Calcitonin seems to be

of limited physiologic significance in humans, at least with regard to

calcium homeostasis. It is of medical significance because of its role as

a tumor marker in sporadic and hereditary cases of medullary thyroid

carcinoma and its medical use as an adjunctive treatment in severe

hypercalcemia and in Paget’s disease of bone. Levels can also be elevated in patients with PHP.

The hypocalcemic activity of calcitonin is accounted for primarily by

inhibition of osteoclast-mediated bone resorption and secondarily by stimulation of renal calcium clearance. These effects are mediated by receptors on osteoclasts and renal tubular cells. Calcitonin exerts additional

effects through receptors present in the brain, the gastrointestinal tract,

and the immune system. The hormone, for example, exerts analgesic

effects directly on cells in the hypothalamus and related structures,

possibly by interacting with receptors for related peptide hormones

such as calcitonin gene–related peptide (CGRP) or amylin. Both of

these ligands have specific high-affinity receptors that share considerable structural similarity with the PTHR1 and can also bind to and

activate calcitonin receptors. The calcitonin receptor shares considerable structural similarity with the PTHR1.

The naturally occurring calcitonins consist of a peptide chain of 32

amino acids. There is considerable sequence variability among species.

Calcitonin from salmon, which is used therapeutically, is 10–100 times

more potent than mammalian forms in lowering serum calcium.

The circulating level of calcitonin in humans is lower than that in

many other species. In humans, even extreme variations in calcitonin

production do not change calcium and phosphate metabolism; no definite effects are attributable to calcitonin deficiency (totally thyroidectomized patients receiving only replacement thyroxine) or excess (patients

with medullary carcinoma of the thyroid, a calcitonin-secreting tumor)

(Chap. 388). Calcitonin has been a useful pharmacologic agent to suppress bone resorption in Paget’s disease (Chap. 412) and osteoporosis

(Chap. 411) and in the treatment of hypercalcemia of malignancy

(see below). However, bisphosphates are usually more effective, and

the physiologic role, if any, of calcitonin in humans is uncertain. On

the other hand, ablation of the calcitonin gene (combined because of

the close proximity with ablation of the CGRP gene) in mice leads to

reduced bone mineral density, suggesting that its biologic role in mammals is still not fully understood.

■ HYPERCALCEMIA

Introduction (See also Chap. 54) Hypercalcemia can be a manifestation of a serious illness such as malignancy or can be detected

coincidentally by laboratory testing in a patient with no obvious illness.

The number of patients recognized with asymptomatic hypercalcemia,

usually hyperparathyroidism, increased in the late twentieth century

when wider testing became readily available.

Whenever hypercalcemia is confirmed, a definitive diagnosis must

be established. Although hyperparathyroidism, a frequent cause of

asymptomatic hypercalcemia, is a chronic disorder in which manifestations, if any, may be expressed only after months or years,

Many

organs Parathyroids

Paracrine Actions Calcium Homeostasis

PTHrP

Brain

Growth Plate

Breast

Smooth muscle

Skin

Kidney

Bone

PTH

Ca2+

FIGURE 410-2 Dual role for the actions of the PTH/PTHrP receptor (PTHR1).

Parathyroid hormone (PTH; endocrine-calcium homeostasis) and PTH-related

peptide (PTHrP; paracrine–multiple tissue actions including growth plate cartilage

in developing bone) use the single receptor for their disparate functions mediated

by the amino-terminal 34 residues of either peptide. Other regions of both ligands

interact with other receptors (not shown).