Interactions of thyroxine and thyrocalcitonin in the rat

Studies of the level of plasma calcium, the hypocalcemic and bone resorptive responses to exogenous thyrocalcitonin (TCT), and the uptake of radiocalcium and radiostrontium by the skeleton and soft tissues were performed in intact and thyroparathyroidectomized rats following a single dose of 250 μg of l-thyroxine (acute T₄) and after 9-21 days of administration of 125 μg of l-thyroxine 3 times a week (chronic T₄). In acute T₄ treated rats, the hypocalcemic effect of exogenous TCT was augmented, whereas, with chronic T₄ treatment, a reduced effect of exogenous TCT was observed. The excretion of ⁸⁵Sr derived from the prelabeled skeleton was reduced with both acute and chronic] T₄ treatment, and exogenous TCT was normally effective in further reducing the excretion of ⁸⁵Sr. Chronic treatment with T₄ caused mild hypocalcemia, and significantly increased skeletal and soft tissue uptakes of 47Ca and ⁸⁵Sr. These results in young rats are dissimilar to previous studies in humans and dogs in which the predominant effect of T₄ is an enhancement of bone resorption, with resultant hypercalcemia, hypercalciuria and demineralization. The data indicate that, in the growing rat, large doses of T₄ have a significant anabolic effect on the calcium metabolism of the skeleton and may promote calcium uptake by certain soft tissues as well.