STUDIES ON PROTEOLYTIC ACTIVITY AND FUNCTION OF THE THYROID GLAND IN RATS ADMINISTERED EXCESS IODIDE

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Abstract
Thyroid auto-proteolytic activity in rats increased significantly only after several weeks of treatment with 1 mg I-/day. Iodide added in vitro did not activate thyroid auto-proteolysis, nor was inhibition observed. Thyroid proteolytic activity was also tested using 131I-labeled heat-inactivated thyroid homogenate from control rats as substrate. Significantly increased proteolytic activity was again demonstrated in thyroid glands from rats on excess I- for several weeks. The increased proteolytic activity is probably not due to a decreased proteolytic resistance of thyroglobulin in the I--treated rats. Increased synthesis or decreased degradation of thyroid proteolytic enzymes is a more likely explanation for this phenomenon. These findings are at variance with those of some previous studies showing decreased proteolytic activity in thyroid glands from rats administered excess I-. Since the rats remained euthyroid the increased auto-proteolysis, which occurred quite late, was considered an unexpected finding and it is at present not certain whether this is related to the successful adaptation. Treatment with I- did not alter the thyroid and hypophyseal weight or histology. Serum free thyroxine level and thyrotropin concentration were also unaffected by excess I-. The adaptation of the intact rat thyroid to prolonged and excess I- intake seems to occur without the assistance of an altered thyrotropin secretion.