Serum calcium decline after intravenous administration of thyrotropin-releasing hormone in man

The influence of iv administration of 0.2 mg thyrotropin-releasing hormone (TRH) on serum calcium was examined in 20 subjects divided into three different groups: one, comprising patients with primary hypothyroidism (A), another, containing euthyroid patients with various diseases (B), and a third, including healthy volunteers (C). Ninety min after the TRH injection total serum calcium (T-Ca) had fallen by 0.19 +/- 0.03 mmol/l in group A (p less than 0.01), by 0.10 +/- 0.02 mmol/l in group B (p less than 0.01), and by 0.08 +/- 0.02 mmol/l in group C (p less than 0.02). Ionized serum calcium (I-Ca) fell in parallel with T-Ca in group A and B. In contrast, serum magnesium was unaffected in all groups. Neither the renal excretion of calcium nor the serum concentration of parathyroid hormone, glucagon or calcitonin changed significantly in response to TRH. These results indicate that TRH has a slight hypocalcemic effect in man which is not caused by plasma dilution, direct influence on the kidneys, or TRH effects on the major calcium regulating hormones. Whether TRH per se, or an increased serum TSH level, induces calcium to leave the vascular space remains to be elucidated.