By Alan Balsam, M.D.
Thorndike Laboratory, Harvard Medical School, and Department of Medicine, Beth Israel Hospital, Boston, Massachusetts 02215
ABSTRACT. Studies were performed to assess the effects of thyroidectomy and hypophysectomy on the overall metabolism of [I25I]T4 and its conversion to[l25I]T3 in slices of rat liver and kidney. Within 2 days after thyroidectomy a slight reduction was evident in the generation of [I25I]T3 from [125I]T4 in both liver and kidney, and 31 days after surgery the activity of this process in these tissues was markedly decreased. Thyroidectomy was associated with diminished disappearance of [I25I]T4, decreased generation of [l25I]iodide and, in kidney, decreased formation of [125I]tetraiodothyroacetic acid. Each of these changes in the metabolism of[I25I]T4 in liver and kidney of thyroidectomized animals was reversed by administration of replacement doses of T4 for 10 days.
Hypophysectomy was similarly associated with diminished conversion of [I25I]T4 to [I25I]T3 that was slight 2 days postoperatively and became more pronounced 31 days after surgery. In liver and kidney from hypophysectomized animals, the degradation of [125I]T4 was significantly reduced and the generation of labeled iodide was unchanged, whereas in kidney, the formation of [l25I]tetraiodothyroacetic acid was markedly reduced. These abnormalities in T4 metabolism in tissues from hypophysectomized animals were completely corrected by hormone replacement with T4. No independent or additional effect was noted when animals were given replacement doses of adrenocortical or gonadal steroids. The present direct studies of the conversion of T4 toT3 in rat liver and kidney in vitro suggest that the impairment in this process observed after thyroidectomy or after hypophysectomy is related to deficiency of thyroid hormone. Moreover, decreased conversion of[125I]T4 to [I25I]T3 in hypothyroidism, at least after thyroidectomy, could not be explained by a more rapid disappearance of [l25I]T3 formed, because the metabolism of [125I]T3 was instead markedly decreased in similar preparations after removal of the thyroid. (Endocrinology 103: 1759, 1978)