Thyroidal Content of Thyrocalcitonin in Hypothyroid and Hyperthyroid Rats

The thyrocalcitonin content of the thyroid glands of euthyroid, hyperthyroid and hypothyroid rats fed a calcium deficient diet was determined in 2 separate experiments by bioassay. Hyperthyroidism was produced by the injection of 50 μg l-thyroxine or 200 mU thyrotropin per day, and hypothyroidism by the dietary administration of 0.05% propylthiouracil or an iodine deficient diet. The log-dose response curve for the hypocalcemic activity of the thyroid homogenates from the euthyroid rats was linear between 0.24 and 0.96 mg thyroid tissue, equivalent to 0.018–0.072 of a single rat thyroid gland. No significant differences in the hypocalcemic response of assay rats was observed when a comparison was made of thyroid tissue homogenates that included parathyroid tissue and thyroid homogenates in which the parathyroid gland was removed prior to tissue homogenization. Thyroxine consistently caused a significant 27–35% reduction in the thyroidal content of thyrocalcitonin, whereas the administration of thyrotropin caused a significant 33% reduction in only 1 of the 2 experiments. Hypothyroidism induced by propylthiouracil or a low iodide diet resulted in a slight increase in thyroidal content of thyrocalcitonin, which was significant in one experiment but no different from the control levels in the second experiment. The results obtained in the rats made hyperthyroid with thyroxine are probably due to an increased recycling of skeletal calcium secondary to the ability of thyroid hormone to accelerate bone resorption rather than a direct effect of thyroid hormone on the secretion of thyrocalcitonin. (Endocrinology81: 256, 1967)