Secretion of 3,3′-Diiodothyronine by the Perfused Canine Thyroid Isolated in Situ*

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Abstract
Previous studies employing perfused dog thyroid lobes have shown that thyroidal secretion is modulated by intrathyroidal deiodination of T4 to T3 and rT3. To characterize further the intrathyroidal iodothyronine-deiodinating processes 3,3′-diiodot;hyronine (3,3′-T2) and T4 were measured in hydrolysate and effluent from isolated dog thyroid lobes during single passage perfusion with a synthetic hormone-free medium. In five experiments the concentrations in effluent from unstimulated thyroid lobes were: 3,3′-T2, 208 ± 62 pmol/liter; and T4, 12.8 ± 2.8 nmol/liter (mean ± SE). During the infusion of 100 μU/ml TSH, there was a parallel increase in the secretion of 3,3′- T2 and T4. After approximately 90 min of TSH infusion, effluent concentrations were: 3,3′-T4, 2490 ± 420 pmol/liter; and T4, 204 ± 24 nmol±liter. In a pronase hydr olysate of the thyroid lobes, the amounts were: 3,3′-T2, 2.44 ± 0.56 pmol/mg; and T4 1070 ± 125 pmol/mg. Expressed as a percent of T4, the amount of 3,3′-T2 in thyroid effluent was approximately 10 times higher than that in thyroid hydrolysate. This relative hypersecretion of 3,3′-T4. was abolished by the infusion of 10-3 or 10-3 M propylthiouracil, an inhibitor of peripheral and intrathyroidal iodothyronine deiodination. Thus, thyroid secretion contains small amounts of 3,3′-T2, most of which seems to originate from intrathyroidal deiodination of T3 and/or rT3.