Direct Measurement ofin VivoSecretion of Pig Thyrocalcitonin by Radioimmunoassay1

The radioimmunoassay for porcine thyrocalcitonin (TC) was used to measure concentrations of the hormone in the thyroid venous effluent blood of young pigs. Thyroid venous blood was continuously and completely collected from the surgically isolated in situ thyroid gland. Control peripheral blood samples were obtained concurrently from a femoral artery. Alterations in the concentration of calcium in blood entering the thyroid gland were achieved by systemic infusions of solutions of CaCl2 or EDTA. Within minutes after exposure of the thyroid gland to hypercalcemia, the concentration of TC in thyroid venous effluent plasma increased from 10–20 to as much as 2000–3000 rg/ml. Hypocalcemia rapidly suppressed high concentrations of TC to undetectable levels. A proportional relationship between concentrations of calcium and TC in thyroid effluent blood was evident above 10 mg Ca/100 ml plasma. With the thyroid venous effluent blood entirely diverted from the systemic circulation, no hormone could be detected in a...