Acute and Longterm Effects of Thyroidectomy on Plasma Calcium in Rats

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Abstract
The acute and long-term effects of thyroidectomy on plasma Ca were studied in male rats bearing parathyroid autotransplants. Zivic-Miller rats were maintained on alternating 12 h light (0800-2000)-12 h dark cycles and fed only during the dark period. They were thyroidectomized at the beginning of the feeding period or at the onset of the period of food deprivation. Wistar rats were thyroidectomized following 24 h starvation, and all animals were bled just before and at timed intervals after surgery. Thyroidectomy resulted in a significant rise in plasma Ca concentration with a maximum increase observed within 3 h. In animals thyroidectomized at the beginning of the feeding period, the maximum increase was 0.94 mg/100 ml of plasma and after 12 h the plasma Ca level was still significantly higher than the presurgery level. When thyroidectomy was performed at the onset of food deprivation, the increase in plasma Ca concentration was 0.84 and 0.57 mg/100 ml at 3 h and 5 h, respectively, but by 12 h plasma Ca was no longer significantly elevated. In Wistar rats starved for 24 h, thyroidectomy raised the plasma Ca level by 0.47 mg/100 ml within 1.5 h, but by 6 h it had decreased to the presurgery level. Two weeks after surgery, the plasma Ca levels of thyroidectomized Zivic-Miller rats were no longer appreciably different from those of control animals during the feeding period. The state of feeding influences the hypercalcemic response of rats to thyroidectomy. Calcitonin is involved in the prevention of postprandial hypercalcemia. A role for the hormone in the maintenance of normocalcemia in the unfed and fasted states cannot be discounted. Animals probably adapt to calcitonin deficiency since the hypercalcemic effect of thyroidectomy was not sustained.

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