3′,5′-Diiodothyronine Release from Perfused Dog Thyroids

Summary

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 μl. 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 and 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 μU/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.

Key-Words:

3′,5′-Diiodothyronine - Radioimmunoassay - Thyroid Secretion