The effect of propylthiouracil on thyroid-stimulating hormone-induced alterations in iodothyronine secretion from perfused dog thyroids

The aim of this study was to see whether the inhibitory effect of propylthiouracil on thyroidal secretion of 3,5,3′-triiodothyronine (T₃) and 3,3′,5′-triiodothyronine (rT₃) could be reproduced in intensively stimulated thyroids, and to elucidate whether an increase in the fractional deiodination of thyroxine (T₄) to T₃ and rT₃ during iodothyronine secretion might be responsible for the transient fall in the T₄/T₃ and T₄/rT₃ ratios in thyroid secretion seen in the early phase after stimulation of thyroid secretion.For this purpose T₄, T₃ and rT₃ were measured in effluent from isolated dog thyroid lobes perfused in a non-recirculation system using a synthetic hormone free medium. 1 mmol/l propylthiouracil induced a significant reduction in thyroid-stimulating hormone (TSH) stimulated T₃ and rT₃ release while the release of T₄ was unaffected. This supports our previous conclusion that T₄ is partially monodeiodinated to T₃ and rT₃ during thyroid secretion. Infusion of 1 mmol/l propylthiouracil for 30 min or 3 mmol/l propylthiouracil for 120 min did not abolish the transient fall in effluent T₄/T₃ and T₄/rT₃ induced by TSH stimulation. Thus, this phenomenon seems not to depend on intrathyroidal iodothyromine deiodinating processes.