Secretion of 3,3'-diiodothyronine by the perfused canine thyroid isolated in situ

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'-diiodothyronine (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 microU/ml TSH, there was a parallel increase in the secreton of 3,3'-T2 and T4. After approximately 90 min of TSH infusion, effluent concentrations were: 3,3'-T2, 2490 +/- 420 pmol/liter; and T4, 204 +/- 24 nmol/liter. In a pronase hydrolysate 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 hypersecrection of 3,3'-T2 was abolished by the infusion of 10(-3) or 10(-5) 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.