The radioimmunoassay of 3,3',5' - triiodothyronine (reverse T3) in unextracted human serum.

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 titre antisera were raised in rabbits using dl=rT3 or l-rT3 conjugated to bovine serum albumin. The 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 sulphonic acid (ANS) in the assay. Mean rT3 levels in healthy euthyroid adults are 0-27 nmo1/1 (range 0-21-1-18); in cord sera 3-67 nmo1/1 (range 2-30-7-45) and in amniotic fluid taken during the second trimester 4-70 nmo1/1 (range 2-22-8-00). In approximately half the hypothyroid subjects, rTO levels were undetectable ( less than0-05 nmo1/1) and in the remaining subjects the mean rT3 level was 0-10 nmo1/1 (range 0-05-0-25).