The Metabolism of 3 : 3′ :5′-Triiodothyronine in Man*

I131-labeled 3:3′:5′-triiodothyronine was administered intravenously to 10 euthyroid hospital patients. Disappearance from the serum and accumulation in the urine were measured. 3:3′:5′-Triiodothyroninc was rapidly degraded and its iodide appeared as free iodide in the serum. The mean serum half-time of the injected compound was of the order of thirty minutes. Within twentyfour hours, more than 50 per cent of the administered labeled iodine was usually eliminated in the urine. Practically all was free iodide. Only a slight amount was excreted as the parent compound. No intermediate degradation products were found in the blood or urine. The rapid metabolic degradation of 3:3′:5′-triiodothyronine makes improbable its existence as a naturally occurring thyroid hormone in man.