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Monodeiodination of Triiodothyronine and Reverse Triiodothyronine During Low and High Calorie Diets^*

Thyroid Research Unit, Endocrine Division, Department of Medicine, University of Geneva Geneva, Switzerland
the Metabolic Unit, Department of Medicine, University of Vermont Burlington, Vermont 05401

Address requests for reprints to: Dr. Albert Burger, Thyroid Research Unit, Endocrine Division, Department of Medicine, University of Geneva, Geneva, Switzerland.

The impact of varying caloric intake on peripheral monodeiodination and plasma disposal of T₃, rT₃, and the three diiodothyronines (T₂) was studied in five normal subjects while they were consuming a low calorie diet (1200 Cal/day) and again while receiving a high calorie diet (3600 Cal/day). Toward the end of each diet period 240 nmol 3,3'-T₂ (126 µg) and 80 nmol 3',5'-T₂ (42 µg) were infused for 7 h, and a bolus injection of 137 nmol 3,5-T₂ (72 µg) was followed by a 12-h infusion of 69 nmol 3,5-T₂ (36 µg) and 111 nmol rT₃ (72 µg) on another day. [¹²⁵I]T₃ (30 µCi) was injected on the third day. The T₂ and rT₃ concentrations were measured by RIA during the 2 days of infusion, and the serum disappearance of [¹²⁵I]T₃ was studied by immunoprecipitation and trichloroacetic acid precipitation of the labeled T₃.

Four to 5% of the plasma disposal of T₃ was accounted for by 3'-monodeiodination, and 36–39% by 5-monodeiodination. Increasing caloric intake resulted in a higher overall plasma disposal rate of T₃, but no change in the percentage of T₃ metabolized by monodeiodination pathways. In contrast, 5%-monodeiodination accounted for 21% of the total plasma disposal of rT₃ during the low calorie diet and 45% during the high calorie intake. This increase in 5%-monodeiodination of rT₃ was at the expense of alternative pathways of disposal. A marked increase in the plasma clearance rate of 3,5-T₂ was also found during the high calorie diet, indicating that the level of caloric intake affects pathways of metabolism other than outer ring monodeiodination.

These studies emphasize the important role played by diet in the regulation of peripheral thyroid hormone metabolism through modulating outer ring monodeiodination, and that overnutrition changes other pathways of iodothyronine metabolism as well.

^* This work was supported by Swiss National Research Foundation Grant 3.943.0.84 (to A.G.B.), NIH Grant AM-18535 (to E.D.), and the University of Vermont General Clinical Research Center (RR-109).

Received October 14, 1986.