Radioimmunological measurements of 3,5-diiodothyronine in hydrolysate and in effluent from perfused canine thyroids

A radioimmunoassay for 3,5-diiodothyronine (3,5-T2) was developed using inner ring labeled 125l-3,5-T2 and a 3,5-T2 antibody produced in rabbits by immunization with 3,5-T2 coupled to human albumin. Separation of free and bound 3,5-T2 was achieved by ascending ion-exchange wick-chromatography. The detection limit was 4 pmol/l. The only important cross-reaction was with T3. It amounted to 0.11 % (mol/mol) and contributed only little to the 3,5-T2 measured in thyroid hydrolysate and thyroid effluent. It was ascertained that no significant loss or generation of 3,5-T2 took place during storage of thyroid effluent samples and during hydrolysis of thyroid homogenate. In four experiments employing eight thyroid lobes, 3,5-T2 was measured in hydrolysate and effluent from perfused dog thyroid lobes during single passage perfusion with a synthetic hormone free medium. During perfusion with control medium 3,5-T2 in effluent was stable around 40 pmol/l. Infusion of 100 microunits/ml TSH induced after a latency period of approximately 20 min a gradual increase in 3,5-T2 release to a level of approximately 180 pmol/l. 3,5-T2 in thyroid hydrolysate was 1.47 +/- 0.05 pmol/mg thyroid wet weight (mean +/- SD). There did not seem to be any major generation of 3,5-T2 by intrathyroidal iodothyronine deiodination. Thus thyroidal secretion of 3,5-T2 is very low and would probably not contribute significantly to the amounts of circulating 3,5-T2.