Medullary thyroid carcinoma and calcitonin.

Medullary carcinoma of the thyroid (MCT) develops from the thyroid C-cells. Thyroid C-cells and MCTs secrete calcitonin (CT), a 32 amino acid polypeptide hormone. The author has described a sensitive direct sequential radioimmunoassay of CT in human serum. The vast majority of healthy subjects had detectable values of serum immunoreactive calcitonin (iCT) and elevated levels were found in patients with MCT. Besides CT, several higher molecular weight substances contribute to CT-immunoreactivity. These substances may represent metabolic products in the processing of a glycosylated procalcitonin to CT. Calcium, several gastrointestinal hormones and ethanol increases CT secretion from normal and neoplastic C-cells, but the physiological regulation of CT secretion has not been firmly established. The author has shown that the levels of serum iCT vary little during day and nighttime. CT acutely reduces bone resorption and, in pharmacological doses, increases urinary electrolyte excretions. A lowering in serum calcium, magnesium and phosphorus levels result but the effect is pronounced only in disease states with a high bone turnover. The physiological role for CT may be preservation of the skeleton at times of increased need for calcium. A causal relationship between postmenopausal osteoporosis and CT deficiency has been proposed. The author has described increased serum 1,25-dihydroxyvitamin D levels and increased trabecular bone remodeling in patients with MCT and normalization of these parameters following surgical cure for MCT. These results were interpreted to indicate that chronic endogenous CT excess directly enhances the renal production of 1,25-dihydroxyvitamin D which, acting synergistically with parathyroid hormone, increases trabecular bone remodeling. Elevated serum iCT is almost invariably found in patients with clinically manifest MCT as well as in several patients with clinically occult MCT. An exaggerated increase in serum iCT levels after provocative testing with pentagastrin and/or calcium can disclose early C-cell neoplasia. Elevated serum iCT may be encountered in non-C-cell neoplasias and in renal insufficiency. Compared to MCT, circulating iCT may show a different immunochemical profile and the response to provocative testing is blunted in these conditions. MCT occurs in a sporadic variety, and in a familial variety as part of two related multiple endocrine neoplasia (MEN) syndromes. MEN IIa consists of MCT and often phaeochromocytomas and/or hyperparathyroidism and invariably exhibits an autosomal dominant mode of inheritance but a variable age of expression.(ABSTRACT TRUNCATED AT 400 WORDS)