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The Effect of Thyroidectomy, Hypophysectomy, and Hormone Replacement on the Formation of Triiodothyronine from Thyroxine in Rat Liver and Kidney^*

Thorndike Laboratory, Harvard Medical School, and Department of Medicine, Beth Israel Hospital Boston, Massachusetts 02215

Address reprint requests to: Dr. Alan Balsam, Division of Endocrinology, Beth Israel Hospital, 330 Brookline Ave., Boston, Massachusetts 02215.

Abstract

Studies were performed to assess the effects of thyroidectomy and hypophysectomy on the overall metabolism of [¹²⁵I]T₄ and its conversion to [¹²⁵I]T₃.i in slices of rat liver and kidney. Within 2 days after thyroidectomy a slight reduction was evident in the generation of [¹²⁵I]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 [¹²⁵I]T₄, decreased generation of [¹²⁵I]iodide and, in kidney, decreased formation of [¹²⁵I]tetraiodothyroacetic acid. Each of these changes in the metabolism of [¹²⁵I]T₄ in liver and kidney of thyroidectomized animals was reversed by administration of replacement doses of Ta for 10 days

Hypophysectomy was similarly associated with diminished conversion of [¹²⁵I]T₄ to [¹²⁵I]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 [12r>I]T₄ was significantly reduced and the generation of labeled iodide was unchanged, whereas in kidney, the formation of [¹²⁵I]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 to T₃ 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 hyroid hormone. Moreover, decreased conversion of [¹²⁵I]T₄ to [¹²⁵I]T₃ in hypothyroidism, at least after thyroidectomy, could not be explained by a more rapid disappearance of [¹²⁵I]Ti formed, because the metabolism of [ltoI]T3 was instead markedly decreased in similar preparations after removal of the thyroid;

Footnotes

^* This research was supported in part by Research Grant AM8416 from the NIAMDD and by Grant RR- 01032 from the General Clinical Research Centers Program of the Division of Research Resources, NIH, Bethesda, Md.

Received January 30, 1978.