Thyrotropin secretion in patients with hyperparathyroidism or hypoparathyroidism: effect of serum calcium on thyrotropin release.

The influence of changes in the serum calcium concentration on TSH secretion was evaluated in patients with primary hyperparathyroidism and idiopathic hypoparathyroidism and in normal subjects. Serum calcium concentrations were 12.7 +/- 0.8, 9.0 +/- 0.4, and 5.7 +/- 0.5 mg/100 ml in hyperparathyroid, normal, and hypoparathyroid subjects, respectively, and were significantly different from each other. Serum T3 and T4 concentrations were comparable among the three groups. The basal serum TSH concentration was highest in hypoparathyroid, lowest in hyperparathyroid, and intermediate in normal subjects. However, all values were within normal limits and were not significantly different from each other. TRH-stimulated TSH secretion was significantly greater in hypoparathyroid patients and significantly less in hyperparathyroid patients than in normal subjects, respectively. The TSH response to TRH was normalized when the serum calcium concentration was normalized by parathyroidectomy in a hyperparathyroid patient or by 1 alpha-hydroxyvitamin D3 administration in a hypoparathyroid patient. To further clarify the mechanism responsible for the modified TSH response to TRH in the hypercalcemic state, rats were made chronically hypercalcemic by the administration of 1 alpha-hydroxyvitamin D3 (0.2 micrograms/100 g BW, ip, for 10 days) and 3% calcium chloride in drinking water. The pituitary TSH content of hypercalcemic rats was significantly greater than that of control rats. The results suggest that decreased TSH secretion produced by chronic hypercalcemia is due to diminished TSH release, rather than to decreased pituitary TSH reserve.