THE RADIOIMMUNOASSAY OF 3,3‘,5’‐TRHODOTHYRONINE (REVERSE T3) IN UNEXTRACTED HUMAN SERUM

Abstract

A specific, sensitive and simple double antibody radioimmunoassay for total serum 3,3'',5'',-triiodothyronine (reverse T3, rT3) in small volumes of unextracted human serum is described. High titer antisera were raised in rabbits using DL-rT3 or L-rT3 conjugated to bovine serum albumin. Selected antisera cross reacted less than 0.003% with triiodothyronine (T3) and 0.14% with thyroxine (T4). A stable high activity rT3 tracer was prepared by iodination of 3,3''-diiodo-L-thyronine by the chloramine-T method, and purified by column chromatography on Sephadex LH-20. Binding of rT3 to endogenous serum proteins was blocked by including 8-anilino-1-naphthalene sulfonic acid (ANS) in the assay. Mean rT3 levels in healthy euthyroid adults are 0.27 nmol/l (range 0.15-0.42); in euthyroid patients 0.29 nmol/l (range 0.11-0.80); in thyrotoxic subjects 1.26 nmol/l (range 0.41-4.66); in T3 thyrotoxic subjects 0.47 nmol/l (range 0.21-1.18); in cord sera 3.67 nmol/l (range 2.30-7.45) and in amniotic fluid taken during the 2nd trimester 4.70 nmol/l (range 2.22-8.00). In approximately 1/2 the hypothyroid subjects, rT3 levels were undetectable (< 0.05 nmol/l, and in the remaining subjects the mean rT3 level was 0.10 nmol/l (range 0.05-0.25).