THE PERIPHERAL METABOLISM OF TRI- AND TETRAIODOTHYROACETIC ACIDS IN MAN*†

Abstract

The metabolism of tracer doses of I131-iabeled 3,5,3[image]-triiodothyroacetic acid (Triac) and tetraiodothyro-acetic acid (Tetrac) was studied in euthyroid subjects and in patients with partial external bile fistulas. In normal subjects, Triac was very rapidly cleared from the blood and concentrated by the liver; both Triac and Triac glucuronide were excreted in high concentrations in the bile. Most of the administered radioactivity rapidly appeared in the urine as inorganic I131. Tetrac was metabolized quite differently. It disappeared from serum more slowly than Triac; an apparent mean half-time in serum of 3.3 days was observed. Tetrac was less highly concentrated in the liver and in the bile. Most of the administered radioactivity ultimately appeared in the urine as inorganic iodide. Formation of conjugates, which appeared to be an important pathway in the metabolism of Triac, was less prominent in the case of Tetrac. These findings are compared with prior studies of the peripheral metabolism of 1-3,5,3[image]-triiodothyronine and l-thyroxine, and are considered in the light of previous observations concerning the binding of these several compounds to plasma proteins.