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

Summary 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-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).