Changes in Serum 3,3′,5′-Triiodothyronine (Reverse T3) Concentrations with Altered Thyroid Hormone Secretion and Metabolism

Abstract

A sensitive radioimmunoassay was developed for measurement of 3,3′, 5′-triiodothyronine (rT₃) in unextracted serum using 8-anilinonaphthalene- 6-sulfonic acid (ANS) to inhibit binding of rT3 to serum proteins. Normal serum r⊂ concentrations were 23 ± 8 (SD) ng/dl. Serum rT₃ concentrations were elevated in patients with hyperthyroidism (90 ± 49 ng/dl), T₃-hyperthyroidism (36 ± 13 ng/dl), pregnant women (49 ± 9 ng/dl) and in cord blood (280 ± 143 ng/dl). Serum rT₃ concentrations were decreased in hypothyroid patients (14 ± 5 ng/dl) and fell during T₃ administration to normals. In hypothyroid patients receiving T₄ replacement, serum rT₃ concentrations varied directly with the dose of T₄ between doses of 0.050 and 0.3 mg/day. The ratio of rT₃ to T₄ was elevated in the sera of hypothyroid and hyperthyroid patients, but was normal in hypothyroid patients receiving T₄ replacement therapy. During propylthiouracil administration, serum rT₃ concentrations were elevated; during methima-zole administration they did not change. In pa-tients with muocardial infraction, the initial mean serum rT₃ concentration was elevated, and it in-creased progressively during the first 5 days of hospitalization. In these patients initial serum T₃ concentrations were low and fell durther over 5 days; serum T₄ concentrations also fell slightly. These data suggest 1) that circulating T₄ is the major source of serum rT₃ but that thyroif secre-tion can sometimes be a significant source, 2) that rT₃ is bound to TBG to a significant degreem and 3) that extrathyroidal metabolism of T₄, T₃ and rT₃ can be altered rapidly by non-thyroidal disease and by propylthiouracil.