A radioimmunoassay of serum 3,5-diiodothyronine.

A radioimmunoassay (RIA) for serum 3,5-diiodothyronine (3,5-T2) was developed using small Sephadex G 25 (fine) columns. Prior to the RIA an alcohol extraction of 3,5-T2 from serum and an evaporation of the extract was performed. The recovery of 3,5-T2 added to serum was in mean (plus or minus SEM) 101 plus or minus 11%. The lower detection limit was 0.012 pmol/column corresponding to 14 pmol/l using 3 ml serum. Due to a 5% cross-reaction of 3,5-T2 antibody with 3,5,3'-triiodothyronine (T3) individual correction for T3 present in serum was necessary. Serum 3,5-T2 levels in 52 éuthyroid controls were (mean plus or minus SD) 105 plus or minus 51 pmol/l. Serum levels were higher in men (125 plus or minus 56 pmol/l. Serum levels were higher in men (125 plus or minus 56 pmol/l) than in women (84 plus or minus 34 pmol/l, P less than 0.005). In 17 hyperthyroid subjects serum 3,5-T2 levels were elevated (232 plus or minus 187 pmol/l, P less than 0.005). In hypothyroid patients and patients with non-toxic goitre serum 3,5-T2 levels did not differ significantly from control values, whereas 8 patients with liver cirrhosis had severely reduced serum 3,5-T2 concentrations (20 plus or minus 23 pmol/l, P less than 0.001).