Comparative Distribution and Fate of I131-Labeled Thyroxine and Triiodothyronine1

Gross and Leblond (1951 a, b) first demonstrated a substance in tissues (unknown 1) which, on the basis of subsequent studies (Gross and Pitt-Rivers 1953 a) apparently was 3,5,3′L-triiodothyronine. Metabolic effects of this material have been investigated in animals, and in normal and myxedematous humans (Asper, Selenkow and Plamondon, 1953, Gross and Pitt-Rivers, 1953 b, Heming and Holtkamp, 1953, Lerman, 1953, Rail et al., 1953, Rawson et al., 1953, and Starr and Liebhold-Schueck, 1953 a & b.). In the athyreotic individual, L-triiodothyrone has been shown to be from 3.5 to 10 times as calorigenic as L-thyroxine in the first few days of therapy. It can produce an increase in metabolic activity in a remarkably short time, effecting clinical improvement within four to six hours (Asper, Selenkow and Plamondon, 1953) in contrast to the delayed metabolic effect and prolonged action of thyroxine. Until labeling of thyroxine with I131 was accomplished (Joliot et al., 1944)