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

SummaryA 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‐l‐naphthalene sulphonic acid (ANS) in the assay. Mean rT3 levels in healthy euthyroid adults are 0.27 nmol/1 (range 015–0.42); in euthyroid patients 0.29 nmol/1 (range 0.11–0.80); in thyrotoxic subjects 1.26 nmol/1 (range 0.41–4.66); in T3 thyrotoxic subjects 0.47 nmol/1 (range 0.21–1.18); in cord sera 3.67 nmol/1 (range 2.30–7.45) and in amniotic fluid taken during the second trimester 4.70 nmol/1 (range 2.22–8.00). In approximately half the hypothyroid subjects, rT3 levels were undetectable (<0.05 nmol/1) and in the remaining subjects the mean rT3 level was 0.10 nmol/1 (range 0.05–0.25).