Although Sandstrom identified and located the parathyroid glands in humans in 1880, there was little investigation into the chemical nature of parathyroid hormone (PTH) before 1959, when extraction and purification procedures were reported and subsequent isolation of a highly purified material resulted in rapid advances in PTH research.1'2 These advances have made it possible to investigate the amino acid sequence of the polypeptide and to develop a radioimmunoassay for the direct determination of small quantities of PTH in biologic fluids.
Chemistry. Bovine, porcine, and human parathyroid hormones have all been isolated and purified. Much of the work on PTH has been done with the bovine hormone, which has been shown to be a single-chain polypeptide consisting of 84 amino acids (Figure 1) with a molecular weight of approximately 9,500. 3'4 Biologically active fragments have been isolated from the amino acid terminal of bovine PTH. Fragments comprising residues 1-29 and 1-34 of the PTH sequence have been shown to possess significant biologic potency, while carboxy terminal fragments have been shown to be devoid of biologic activity.5
A biosynthetic precursor, proparathyroid hormone (pro-PTH), has been identified in bovine and human parathyroid tissue.6'7'8 The biologic activity of bovine pro-PTH has been shown to be only one third of native PTH. ProPTH comprises only three per cent of the total immunoreactive parathyroid hormone (IPTH) in bovine tissue extracts. However, in certain human parathyroid adenomas it has been shown that pro-PTH constitutes up to 15 per cent of the total immunoreactive hormone, whereas in others it is undetectable.
After completion of synthesis, proPTH apparently undergoes rapid cleavage to PTH. This accounts for the low content of pro-PTH in normal parathyroid glands. The chief secretory product of the parathyroid gland is PTH (W 9,500). After secretion PTH is rapidly cleaved to smaller fragments at unknown peripheral sites. Much of the IPTH peptides in the peripheral circulation of humans and cattle is a carboxy terminal fragment with a molecular weight of 6,000 to 7,500.*
Regulation of parathyroid hormone secretion. The factor of prime importance in controlling the secretion of PTH is the ionized calcium concentration of the plasma. Through a negative feedback control mechanism, a fall in plasma calcium (Ca) stimulates PTH secretion. In humans, lowering of the plasma calcium by 1.5 mg./ 100 ml. leads to an increase in PTH concentration that may be as high as 400 per cent. The half-life of the hormone in blood is estimated to vary between 20 and 30 minutes. The short half -life and the rapid secretion of the hormone in response to changes in blood calcium indicate that rapid changes in hormone secretion play an important role in the minute-to-minute regulation of blood calcium. The concentration of phosphate (P) in the plasma has no direct regulatory influence on hormone secretion but, when increased, leads to stimulation by virtue of its ability to lower plasma calcium.
Parathyroid hormone actions. The prime function of PTH is to maintain extracellular calcium ion activity within physiologic limits. The hormone accomplishes this by increasing bone resorption, by increasing the renal tubular resorption of calcium, and by increasing the gastrointestinal absorption of Ca. As a result of PTH-induced bone resorption, calcium and phosphate are mobilized from bone into extracellular fluid; organic matrix as well as mineral is broken down during the bone resorptive process. Another major effect of PTH is to stimulate phosphate excretion by the kidney.
Radioimmunoassay of parathyroid hormone. The most sensitive method for measuring PTH is by radioimmunoassay. In the immunoassay measurement of human PTH in most laboratories, the reference standard has been either purified bovine hormone or human hyperparathyroid serum; with the latter the concentration of IPTH in unknown samples is expressed as microliter equivalents of the standard hyperparathyroid serum per milliliter (mclEq. /ml.).
Figure 1. Amino acid sequence of bovine parathyroid hormone.3,4
Heterogeneous forms of PTH are present in the peripheral circulation.10'11 This is not surprising, considering the evidence for pro-PTH and for PTH fragments. Recently, radioimmunoassays have been developed that specifically measure the amino terminal and carboxy terminal of bovine PTH. By using these antisera in radioimmunoassays, it has been demonstrated that the concentration of intact PTH constitutes less than 15 per cent of the total immunoreactive hormone in the circulation of patients with parathyroid adenomas. The remaining 85 to 90 per cent of the immunoreactive hormone consists of a carboxy terminal hormone fragment with a molecular weight of 6,000 to 7,500.
Recent evidence suggests that a biologically active amino-terminal PTH fragment (MW 4,500 or less) may be present in small concentrations in the peripheral circulation of humans.12 Though the predominant form of IPTH in the peripheral circulation is a biologically inactive carboxy terminal fragment, immunoassay measurements correlate well with the day-to-day overall secretory activity of the parathyroid glands.
The antisera used in our routine assay detects the native hormone (MW 9,500) and the predominant circulating carboxy terminal fragment (MW 6,0007,50O).13 As seen in Figure 2, the assay is an excellent tool for evaluation of patients with various disorders of parathyroid function. With our assay the normal range of values in children varies from undetectable to 10 mclEq. / ml., with no significant differences in levels in the different age groups. However, in a study of normal subjects six months to 20 years of age, Arnaud et al.14 reported that mean serum IPTH levels were generally higher in youngest children, decreased to a nadir at seven to nine years of age, and then increased gradually to plateau at adolescence.
NEONATAL PARATHYROID FUNCTION
We have conducted studies of parathyroid function in normal and hypocalcémie newborn infants,15 the results of which are summarized below.
Normal infants. In the cord blood, elevated levels of plasma calcium were observed, while levels of serum IPTH were either undetectable or low. In normal newborns there was a decrease in plasma calcium during the first 48 hours of life, but the serum IPTH level in most samples remained undetectable or low. After 48 hours of life there were parallel increases in plasma calcium and serum IPTH levels.
Hypocalcemic infants. In the majority of hypocalcémie infants studied there was a history of abnormal pregnancy, delivery, or neonatal course. Most of the infants were younger than 96 hours when hypocalcemia (plasma calcium less than 7.5 mg. /100 ml.) was discovered. It is of interest that most of the hypocalcémie infants had undetectable or inappropriately low serum IPTH levels. This is in contrast to the consistently elevated levels of serum IPTH that we observed in older infants and children with hypocalcemia due to a variety of causes other than parathyroid insufficiency (Figure 3).
Evidence obtained from these studies indicates that parathyroid secretion is normally low in the early newborn period and that impaired parathyroid function, characterized by undetectable or low serum IPTH, is present in most infants with neonatal hypocalcemia. Since hypocalcemia may not be present in some newborn infants with low or undetectable levels of serum IPTH, other nonparathyroid factors appear to contribute to the lowering of plasma calcium in the neonatal period. The net effect of unknown serum hypocalcemic factor(s) on the one hand and parathyroid activity on the other may account for the differences in plasma calcium levels observed between normal and hypocalcémie infants.
Idiopathic hypoparathyroidism in childhood may be divided into two general classes - early onset (infantile) and later onset (juvenile).16'17 Early onset. In the infantile form, symptoms appear during the first year of life, and in reported cases the incidence of the disorder is twice as high in males as in females. Two types of infantile hypoparathyroidism have been described - a form that occurs in males as a sex-linked Mendelian recessive trait and a sporadic form present in both males and females.
Figure 2. Serum IPTH levels in normal subjects and in patients with primary hyperparathyroidism, chronic renal insufficiency, and hypoparathyroidism. 13
The sex-linked form of the disease has a relatively benign prognosis, while the form that occurs sporadically in either sex has a relatively poor outlook. The poor prognosis in the sporadic infantile variety is the result of severe infection with diarrhea and severe associated anomalies. In addition to absence or hypoplasia of parathyroid glands, infants. with the sporadic form have hypoplasia or absence of the thymus, which is believed to account for the severe infection in this group. Because the superior parathyroids are derived from the fourth pharyngeal pouch and the inferior thyroids and thymus from the third pharyngeal pouch, the sporadic form of infantile hypoparathyroidism has been labeled the III and IV pharyngeal pouch syndrome.
Figure 3. Comparison of IPTH values in hypocalcémie newborn infants and hypocalcémie older children. Shaded area indicates normal range. Broken line indicates limits of detectability of parathyroid hormone Immunoassay. 15
Later onset. Primary hypoparathyroidism seen in patients over one year of age occurs with equal frequency in males and females. Two clinical forms have been recognized - hypoparathyroidism that occurs in association with moniliasis and Addison's disease and isolated primary hypoparathyroidism.
The syndrome of hypoparathyroidism, Addison's disease, and superficial moniliasis is often familial; however, sporadic cases have been reported. The onset of hypoparathyroidism frequently precedes that of adrenal insufficiency, sometimes by several years. Adrenal insufficiency may have an ameliorating effect on hypoparathyroidism. Conversely, it is important to be aware that exacerbations of the manifestations of parathyroid insufficiency may occur following hormonal treatment of adrenal insufficiency.
The etiology of this syndrome is unknown, although there is some evidence suggesting that it is an autoimmune disorder. Death may result from adrenal insufficiency or from severe infection. At autopsy, parathyroid tissue is either not demonstrable or is diminished and is replaced by fat; the adrenal glands are small due to cortical atrophy.
A child with hypoparathyroidism frequently is brought to the attention of a physician because of neurologic symptoms. Convulsive seizures and increased neuromuscular irritability with signs and symptoms of tetany its equivalent may be prominent features of hypoparathyroidism. The symptoms include paresthesias, facial grimacing, muscle cramps, muscle rigidity, laryngeal stridor, and carpopedal spasm. Chvostek's and Trousseau's signs frequently are positive. Ocular manifestations (conjunctivitis and corneal ulcerations), alopecia, and dermal changes (thick, dry skin) also may occur in hypoparathyroidism.
Roentgenograms of the skeletal system may reveal no significant changes from normal, although both decreased and increased density have been recorded. The reason for these differences is obscure.
Characteristic laboratory findings include reduced plasma calcium (as low as 4 mg. /100 ml. or less), elevated plasma phosphate, and inappropriately low or nondetectable serum IPTH. The finding of a low or nondetectable serum IPTH in association with hypocalcemia is highly significant and indicates impaired or absent parathyroid gland function. In the presence of normal parathyroid function, hypocalcemia is associated with elevated rather than low serum IPTH.
Pseudohypoparathyroidism. Idiopathic hypoparathyroidism must be differentiated from pseudohypoparathyroidism, which is a syndrome in which the chemical derangements (hypocalcemia and hyperphosphatemia), although characteristic of hypoparathyroidism, are refractory to treatment with PTH. This syndrome is a familial hereditary disorder usually accompanied by distinctive clinical features including short stature, bone deformities, round face, subcutaneous ossification or calcification or both, and mental retardation.
End organ refractoriness to PTH appears to be the underlying defect in pseudohypoparathyroidism. Thus, while parathyroid extract increases the blood calcium level and promotes the urinary excretion of phosphate and cyclic adenosinemonophosphate (AMP) in patients with hypoparathyroidism, one or more of these responses are blunted in patients with pseudohypoparathyroidism. Moreover, elevated levels of circulating IPTH are found in patients with pseudohypoparathyroidism, in contrast to low or undetectable levels in patients with idiopathic hypoparathyroidism.
Treatment of idiopathic hypoparathyroidism consists of daily use of either calciferol (vitamin D2) or dihydrotachysterol (AT 10) with or without calcium supplements. The maintenance dose of calciferol may range from 25,000 to 150,000 units/day; the maintenance dose of AT 10 is usually one third that of calciferol. The aim of therapy is to achieve a normal blood level of calcium while avoiding hypercalcemia and hypercalciuria.
Primary hyperparathyroidism. Primary hyperparathyroidism is an uncommon disorder in childhood. As in adults, the largest number of cases have been associated with a single adenoma, most commonly of the chief cell type; in approximately 20 per cent of children with the condition it is due to diffuse hyperplasia. There are several reports of hereditary or familial hyperparathyroidism.
The symptoms of primary hyperparathyroidism in childhood tend to be chronic with remissions and exacerbations. Symptoms are related to the skeletal system, kidneys, and a variety of manifestations that occur secondary to hypercalcemia.
Skeletal involvement is quite common and includes bone pain, tenderness, and an abnormal gait. In more severe cases there may be stunted growth and bowing of the legs. Roentgenograms frequently reveal generalized osteoporosis with thinning of the cortex and trabeculae. Local skeletal changes are observed in approximately 50 per cent of children with this condition. These include disappearance of the lamina dura, subperiosteal erosions of cortical bone of the phalanges, fuzzy moth-eaten calvarium, and single or multiple bone cysts.
Renal signs and symptoms include nephrocalcinosis, nephrolithiasis, renal colic, hematuria, pyuria, and ureteral obstruction. Renal lithiasis has been reported in approximately 25 per cent of children with primary hyperparathyroidism. Polyuria and polydipsia associated with hypercalcemia also are observed in about 25 per cent of children with primary hyperparathyroidism.
Hypercalcemia, which exerts a depressant effect on neuromuscular irritability, may induce muscle hypotonicity and weakness as well as gastrointestinal atony with nausea, vomiting, and constipation. Alterations in mood, behavior, and mental function may be noted.
Hypercalcemia is usually, but not invariably, present in children with primary hyperparathyroidism. In a recent excellent review it was reported that in all but three of 38 children with primary hyperparathyroidism the plasma calcium level was 12 mg. /100 ml. or greater.18 Occasionally, however, juvenile hyperparathyroidism may be associated with an equivocal or minor elevation of plasma calcium. The plasma inorganic phosphate concentration is characteristically low (less than 3.0 mg. /100 ml.) or low normal. Serum alkaline phosphatase may be elevated. Hypercalciuria frequently can be demonstrated. Urinary hydroxyproline (reflecting breakdown of organic matrix of bone) and urinary cyclic 3 ',5' AMP excretion may be increased.
The finding of an elevated serum IPTH level in association with hypercalcemia is highly suggestive, though not diagnostic, of primary hyperparathyroidism (Figure 2). Hypercalcemia with an elevated serum IPTH occasionally may be found in certain patients with nonparathyroid tumors that secrete a parathyroid-like substance and possibly may be found in some patients with secondary hyperparathyroidism associated with chronic renal disease.
The definitive therapy for primary hyperparathyroidism is surgery.
Infantile hyperparathyroidism. Although it is unusual, primary hyperparathyroidism may be observed in the newborn period. Two forms have been described.19'20 One form is hyperparathyroidism in infants born to mothers with hypocalcemia associated with maternal hypoparathyroidism. At birth the infants may appear normal and usually have normal or depressed plasma calcium levels, although roentgenograms of bone demonstrate changes consistent with hyperparathyroidism. It is postulated that, due to exposure to maternal hypocalcemia, fetal parathyroid hyperplasia develops which leads to increased fetal bone resorption. Following birth, the infant skeleton avidly takes up calcium, and this accounts for the depressed or normal plasma calcium levels. With time, the parathyroid hyperplasia subsides and the bone lesions heal spontaneously.
The second form is infantile hereditary hyperparathyroidism. It occurs in infants in whom there is no history of parental parathyroid disease and appears to be of autosomal recessive inheritance.
Characteristically, these infants have severe hypercalcemia (15 to 30 mg. /100 ml.), skeletal demineralization, and, frequently, renal calcinosis. Common symptoms include poor feeding, constipation, respiratory difficulty, and hypotonia. Hereditary parathyroid hyperplasia is considered a surgical emergency since the disease progresses and death occurs in early infancy.
Secondary hyperparathyroidism. Evidence of increased activity of the parathyroid glands exists in a number of clinical disturbances, including chronic renal disease, vitamin D-deficient rickets, and the malabsorption syndromes. The increased parathyroid activity is compensatory in nature and results from the tendency towards plasma calcium deficiency that is characteristic of these disorders. The occurrence of secondary hyperparathyroidism in all of these conditions serves to alleviate or prevent hypocalcemia.
In general, the highest levels of serum IPTH have been observed in patients with chronic renal insufficiency, in which depressed intestinal calcium absorption associated with acquired resistance to vitamin D has been well established. As the plasma calcium increases in chronic renal disease, either through vitamin D therapy, calcium infusion, or hemodialysis, the serum IPTH level decreases.
MAGNESIUM AND PARATHYROID FUNCTION
Recent evidence indicates that magnesium (Mg) is intimately involved in parathyroid function. Magnesium deficiency may occur in a number of entities, including gastrointestinal loss of fluids, malabsorption syndromes, and primary hypomagnesemia. In the latter syndrome there is an isolated defect in the gastrointestinal transport of magnesium.
It is known that hypocalcemia frequently accompanies magnesium deficiency. Characteristically, this form of hypocalcemia is resistant to calcium therapy but responds rapidly to the administration of magnesium salts. There are some studies, especially in chronic alcoholics, that indicate that patients with magnesium deficiency are resistant to the action of PTH.21 In other studies of magnesium deficiency, however, end organ resistance to PTH has not been demonstrated.
We recently studied a young woman with magnesium deficiency and hypocalcemia in whom normal end organ responsiveness to PTH was demonstrated.22 Of particular interest in this patient was the fact that, in the presenee of hypocalcemia and hypomagnesemia, baseline serum IPTH levels were nondetectable to low (Figure 4). The administration of an oral phosphate load resulted in further depression of plasma calcium and magnesium but did not elicit an increase in serum IPTH. The intramuscular administration of magnesium sulfate resulted in parallel increases in serum IPTH, calcium, magnesium, and renal phosphate clearance. Several weeks after magnesium therapy was discontinued hypomagnesemia, hypocalcemia, and depressed serum IPTH levels were again observed. Similar findings have been reported by Suh et al.23 in a young boy with primary hypomagnesemia.
Figure 4. Serum IPTH, total calcium, ionized calcium, magnesium, and renal phosphate clearance (Pc) before and after an oral phosphorus load and intramuscular magnesium sulfate (I. M. MgSO/i) in a patient with primary hypomagnesemia. The horizontal shaded areas indicate normal ranges. Normal ranges for serum total calcium, ionized calcium, and magnesium are defined as + 2 S.D. of mean normal values established in our laboratory. Normal range of serum IPTH extends from nondetectable to + 2 S.D. of mean. Broken line indicates limits of detectability of the parathyroid hormone immunoassay. 22
The results of these studies indicate that the synthesis or secretion or both of PTH may be impaired in the magnesium-deficient state in humans. The possibility of magnesium depletion should be considered in patients with unexplained hypocalcemia. Correction of magnesium depletion with either parenteral or oral magnesium supplements restores normal parathyroid function.
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