Thyroxine entering the thyroid gland via the vascular bed may leave the gland as triiodothyronines. Studies with perfused dog thyroid lobes.

In perfused dog thyroid lobes some of the T4 released from thyroglobulin (endogenous T4) is deiodinated during the secretion process to T3 and rT3, leading to a relative hypersecretion of triiodothyronines. The enzymes responsible for this deiodination are found in the follicular cells and resemble the T4 deiodinases in the liver. In the present study we investigated whether exogenous T4 infused into the thyroid via the vascular bed is also deiodinated to T3 and rT3. In two-sided thyroid perfusion experiments one lobe received T4 (200 ng/ml) during the perfusion period of 60-200 min. The contralateral lobe acted as control. T4 infusion was followed by a gradual increase in T3 and rT3 in thyroid effluent to a new level. This corresponded to 3.4 +/- 1.0% (mean +/- SE, n = 6) of the infused T4 leaving the gland as T3 and 0.7 +/- 0.2% as rT3. This thyroidal deiodination of exogenous T4 to T3 and rT3 was significantly reduced if T4 was infused in iodothyronine-free dog serum instead of buffer medium, probably owing to the presence of T4-binding proteins. Serum perfusion did not affect the deiodination of endogenous T4 to T3 significantly, whereas the deiodination of endogenous T4 to rT3 was depressed. Hence both endogenous T4 (from thyroglobulin) and exogenous T4 (arriving via the vascular bed) are deiodinated to triiodothyronines in the thyroid. Intrathyroidal T3 generation from plasma-borne T4 could be of considerable quantitative importance in patients with hyperstimulated goiters.