Circulating reverse triiodothyronine (rT3) in the dog

Plasma reverse triiodothyronine (rT₃) concentration was measured by radio immunoassay (RIA) in a group of 15 dogs. The mean rT₃ concentration was 187 ng/100 ml which was 3 times higher than radioimmunoassayable triiodothyronine (T₃) concentration. rT₃ measurement in thyroid and peripheral venous plasma in 3 dogs showed that the unusually high circulating rT₃ levels in this species could not be explained on the basis of augmented thyroidal rT₃ secretion. Study of rT₃-protein binding by equilibrium dialysis also failed to show any evidence of unusual rT₃-protein interaction (rT₃ free fraction was 2 — 3 times greater than in normal human serum). Among all the species examined so far (man, monkey, sheep, dog and rat), only in the dog are the circulating rT₃ levels significantly higher than T₃ suggesting that in this species the 5-deiodination, in marked contrast to the 5'-deiodination noted in several other species, is a major pathway normally involved in the initial monodeiodination of T₄.