Familial pseudohypoparathyroidism. Role of parathyroid hormone and thyrocalcitonin

C-Cell Hyperplasia and Cystic Papillary Thyroid Carcinoma in a Patient with Type 1B Pseudohypoparathyroidism and Hypercalcitoninaemia: Case Report and Review of the Literature

Hypercalcitoninaemia has been described in patients with pseudohypoparathyroidism (PHP) type 1A and 1B. Elevated calcitonin levels are thought to result from impaired Gsα receptor signaling, leading to multiple hormone resistance. Evidence on the risk of medullary thyroid carcinoma (MTC) or C-cell hyperplasia in PHP patients with hypercalcitoninaemia is lacking. A 43-year-old Caucasian man was referred to our endocrinology clinic for chronic hypocalcemia associated with elevated serum parathormone levels and a single cystic thyroid nodule. The patient did not show skeletal deformities, and screening for concomitant hormone resistances was negative, except for the presence of elevated serum calcitonin levels. The workup led to a molecular diagnosis of sporadic PHP1B. Fine needle aspiration of the thyroid nodule was not diagnostic. The calcium stimulation test yielded an abnormal calcitonin response. Given the scarcity of data on the risk of thyroid malignancy in PHP and calcium stimulation test results, total thyroidectomy was performed. Histological examination revealed cystic papillary thyroid cancer in a background of diffuse C-cell hyperplasia. To our knowledge, we are the first to describe a rare form of thyroid cancer combined with C-cell hyperplasia in a patient with PHP and hypercalcitoninaemia. In the present case, a mere receptor resistance might not fully explain the elevated calcitonin levels, suggesting that hypercalcitoninaemia should be carefully evaluated in PHP patients, especially in the case of concomitant thyroid nodules. Further studies on larger cohorts are needed to elucidate this topic.

Familial Pseudohypoparathyroidism Presenting in Adult Life1

Three members of one family who presented in adult life with pseudohypoparathyroidism are reported. The importance of screening members of the family of patients with pseudohypoparathyroidism is stressed. Higher doses of 1α-hydroxycholecalciferol are required than in idiopathic or surgical hypoparathyroidism or in chronic renal failure.

Hypoparathyroidism and pseudohypoparathyroidism in childhood

Although relatively uncommon, the conditions of hypoparathyroidism and pseudohypoparathyroidism in childhood provide an exciting diagnostic and therapeutic challenge. Knowledge of calcium-phosphate homeostasis has progressed rapidly over the past few years so that our understanding of the basic pathophysiological mechanisms has increased tremendously. However, further clinical and basic scientific research will, no doubt, unravel further variations of the various disease entities described.

Hypoparathyroidism

Recent advances in our understanding of the physiologic actions of PTH and vitamin D have clarified certain aspects of the pathogenesis, classification, and management of hypoparathyroidism. Central to pathogenesis and categorization is the recognition that hypoparathyroidism may result from PTH deficiency, ineffectiveness, or resistance, with a resultant inability to stimulate adenylate cyclase in target tissues. This aberration in adenylate cyclase activity impairs certain physiologic responses such as renal phosphate excretion and renal calcium reabsorption that are required for proper calcium homeostasis. Also critical is the subnormal production of 1 alpha,25-dihydroxycholecalciferol (1,25-DHCC). Although the precise mechanism for the deficiency of 1,25-DHCC remains unclear, one may hypothesize that in hormone-deficient or hormone-ineffective hypoparathyroidism, decreased synthesis results from the absence of the two recognized stimuli for 1 alpha-hydroxylase--bioactive PTH and hypophosphatemia. Provision of either one of these stimuli would then be expected to restore 1,25-DHCC to normal levels, which could explain the calcemic response to PTH in these patients. There is some evidence that the synthesis of 1,25-DHCC may be "primarily" affected in PTH-resistant hypoparathyroidism, and thus may be unresponsive to any of the known stimuli. It remains conceivable, however, that during normocalcemic phases, such patients may improve their renal cyclic AMP and phosphaturic responses to PTH, with associated improvement in 1,25-DHCC synthesis. Certain acquired forms of PTH resistance such as hypomagnesemia and end-stage renal disease may also be associated with defective 1-hydroxylation. Whether occurring primarily or as a secondary process, the subnormal production of 1,25-DHCC may influence calcium and skeletal metabolism directly or by modifying response to PTH. The availability of 1,25-DHCC provides an effective and physiologically meaningful mode of therapy for most cases of hypoparathyroidism.

The syndromes of primary hormone resistance

The clinical features, genetics, pathophysiology, and management of endocrine diseases in which primary hormone resistance is the fundamental defect have been reviewed. Primary hormone resistance has been documented for nearly all hormones--vasopressin, parathyroid hormone, growth hormone, adrenocroticotropin, thyrotropin, gonadotropins, insulin, androgens, cortisol, aldosterone, progesterone, thyroid hormones, and vitamin D. A striking exception is estradiol, a steroid that may be vital for early embryonic development. Most of the hormone unresponsiveness syndromes represent only partial defects, and it is likely that most such patients go unrecognized. Therefore, hormone resistance should be suspected not only when a patient presents with hypofunction of particular endocrine system combined with high endogenous hormone levels but also whenever apparently normal function of an endocrine system is associated with inappropriately elevated levels of the corresponding hormone. The value of these defects in hormone responsiveness as a natural laboratory for the study of the normal mechanisms of hormone action is discussed.

Recent advances in calcium metabolism. II. Disorders of calcium homeostasis

Abnormalities of calcium and mineral metabolism are described in relation to the differential diagnosis, clinical characteristics, diagnostic procedures, and treatment of infants and children with hypocalcemia, hypercalcemia, rickets, chronic renal insufficiency, and other disorders of calcium metabolism. Understanding of the basic pathogenesis of each abnormality of calcium homeostasis is essential for the rational management of affected patients.

Regulation of circulating parathyroid hormone levels: normal physiology and consequences in disorders of mineral metabolism

A review of the chemistry, the biosynthesis, the regulation of the secretion and the metabolism of parathyroid hormone (PTH) provides a background to problems related to the determination of immunoreactive PTH levels in the blood of normal subjects and of patients suffering from disorders of mineral metabolism. Immunoreactive PTH measurements have to be interpreted in the light of the immunoheterogeneity encountered and the occurrence of several circulating PTH species with different molecular weights and immunological properties.

Pseudohypoparathyroidism. Disappearance of the resistance to parathyroid extract during treatment with vitamin D

Serum immunoreactive parathyroid hormone (iPTH) levels were increased in a 15 year old girl with pseudohypoparathyroidism, hypocalcemia, hyperphosphatemia, and minimal phosphaturic and absent hypercalcemic responses to exogenous parathyroid extract (PTE). Following normalization of the serum calcium concentration with vitamin D, serum iPTH and phosphate concentrations returned to the normal range, and phosphaturia could be clearly stimulated and hypercalcemia induced by PTE. On the other hand, the urinary cyclic adenosine 3',5'-monophosphate (cyclic AMP) excretion could not be stimulated, suggesting that in this case, there appears to be no relationship between the urinary excretion of cyclic AMP and the phosphaturic effect of PTE. The minimal phosphaturic effect and the lack of hypercalcemic effects of PTE in untreated pseudohypoparathyroidism can be explained by the secondary hyperparathyroidism causing elevated iPTH levels rather than by a defect at the level of the receptor sites. A requirement of pharmacologic amounts of vitamin D per se, however, for the responsiveness of patients with pseudohypoparathyroidism to PTE cannot be ruled out.

Glucose tolerance and insulin secretion in patients with parathyroid disorders. Effect of serum calcium on insulin release

To evaluate the role of serum calcium in human insulin secretion, insulin responses after a 100-g oral glucose load were studied in nine patients with primary hyperparathyroidism, five with idiopathic hypoparathyroidism, three with pseudohypoparathyroidism and one with normocalcemic secondary hyperparathyroidism. Glucose tolerance values in these disorders were almost normal. Insulin responses in primary hyperparathyroidism were increased, and those in idiopathic hypoparathyroidism and pseudohypoparathyroidism were reduced significantly as compared to normal subjects. Isulin response in secondary hyperparathyroidism was normal. The calculated insulin area during an oral glucose load was significantly correlated with serum calcium (5.1 to 12.2 mg per deciliter), and a linear relation was obtained (y = 1.59x - 3.3, r = 0.81, p less than 0.001), although a relation with the glucose area was not found. These observations indicate that serum calcium has an important effect on insulin secretion in parathyroid disorders.

Pseudohypoparathyroidism: defective excretion of 3',5'-AMP in response to parathyroid hormone

Urinary excretion of cyclic adenosine 3',5'-monophosphate (3',5'-AMP) was tested in normal subjects and patients with pseudohypoparathyroidism, idiopathic hypoparathyroidism, surgical hypoparathyroidism, and pseudopseudohypoparathyroidism under basal conditions and after a 15 min infusion of purified parathyroid hormone. Basal excretion of the nucleotide was less than normal in the patients with hypocalcemic disorders and greater than normal in pseudopseudohypoparathyroidism. Parathyroid hormone caused a marked increase in excretion of 3',5'-AMP in all subjects except those with pseudohypoparathyroidism; nine patients with this disorder did not respond to the hormone and four showed a markedly deficient response. Radioimmunoassay showed that parathyroid hormone circulated in increased amounts in plasma from patients with pseudohypoparathyroidism and became undetectable when serum calcium was increased above 12 mg/100 ml. Suppression of parathyroid hormone secretion by induction of hypercalcemia did not alter the deficient response to exogenous hormone. The results indicate that: (a) parathyroid hormone circulates in abnormally high concentrations in pseudohypoparathyroidism and secretion of the hormone responds normally to physiological control by calcium; (b) testing urinary excretion of 3',5'-AMP in response to infusion of purified parathyroid hormone appears to be an accurate and sensitive index for establishing the diagnosis of pseudohypoparathyroidism; and (c) the metabolic defect of the disorder can be accounted for by a lack of or defective form of parathyroid hormone-sensitive adenyl cyclase in bone and kidney.