Radioimmunological Measurements of 3,5-Diiodothyronine in Hydrolysate and in Effluent from Perfused Canine Thyroids

Summary

A radioimmunoassay for 3,5-diiodothyronine (3,5-T₂) was developed using inner ring labeled ¹²⁵I-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 μU/ml 7SH 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 ± 0.50 pmol/mg thyroid wet weight (mean ± SD). 7here 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.

Key-Words:

3,5-T₂ - Propylthiouracil - Iodothyronine Deiodination - Thyroid Secretion