3',5'-diiodothyronine release from perfused dog thyroids

A new radioimmunoassay for 3',5'-diiodothyronine (3',5'-T2) is described. The detection limit was 1-2 pmol/l with a sample size of 200 microliters. Cross-reaction with T4, T3, and rT3 was 0.005, 0.003, and 1.3% respectively. The intra- and interassay coefficients of variation were 5.1% and 9.2%. Dilution curves of effluent from perfused thyroid lobes of thyroid hydrolysate were parallel to the standard curve. By addition of 3',5'-T2 or rT3 standards to thyroid effluent sampling tubes or to thyroid homogenate before hydrolysis it was ascertained that no artifactual loss of 3',5'-T2 or generation of 3',5'-T2 from rT3 was taking place during collection and storage of thyroid effluent or during hydrolysis of thyroid homogenate. 3',5'-T2 was measured in effluent from perfused thyroids and in hydrolysate of the thyroids in six mongrel dogs. Only minute amounts of 3',5'-T2 was found: 1.66 +/- 0.41 pmol/mg thyroid wet weight (mean +/- SD) in hydrolysate, around 20 pmol/l in thyroid effluent during control perfusion and around 170 pmol/l during prolonged infusion of 100 micro U/ml bovine TSH. By measuring the apparent 3',5'-T2 concentration in perfusion medium with added T4 and rT3 standard corresponding to the amounts present in thyroid effluent samples, and dilutions of thyroid hydrolysates it was excluded that T4 and rT3 interference could account for more than maximally 20% of the measured 3',5'-T2. Thus 3',5'-T2 is present in the thyroid and secreted from the thyroid, but only in very small amounts.