Effect of Hypocalcemia on Hormonal Responses to Thyrotropin-Releasing Hormone

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Abstract
The effects of chronic hypocalcemia on pituitary and thyroidal responses to TRH [thyrotropin releasing hormone] were studied in 6 euthyroid and 3 mildly hypothyroid subjects with surgical hypoparathyroidism or pseudohypoparathyroidism. In the euthyroid patients, increments in serum TSH [thyrotropin]. T3 [triiodothyronine] and T4 [thyroxine] were equal under hypocalcemic or normocalcemic conditions; however, a slightly but significantly lower basal serum T3 during hypocalcemia in conjunction with unaltered basal serum T4 and reverse T3 levels suggests that hypocalcemia may alter peripheral metabolism of T4 and/or T3. Hypothyroid subjects showed more variable responses to hypocalcemia; after restoration of normocalcemia, 2 patients demonstrated greater serum T3 increments following TRH and lower TSH levels, suggesting an impairment of thyroidal secretion by hypocalcemia, while the 3rd showed no effect of hypocalcemia on TSH, T3 or T4 responses to TRH. Hypocalcemia had no effect on prolactin responses to TRH in the combined group of euthyroid and hypothyroid patients. Since only minor effects on basal or TRH stimulated hormone levels were seen in all patients, hypocalcemia rarely has clinically significant effects on pituitary secretion of TSH and prolactin and thyroidal release of T4 and T3.

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