抄録
The effect of propylthiouracil (PTU), methylthiouracil (MTU), mercaptoimidazole and potassium iodide was studied on the metabolism of thyroxine (T4), and the following results were obtained :
1) Urinary excretion rate of I131 decreased and fecal radioactivity increased after I131-T4 injection in thyroidection in thyroidectomized rats treated with PTU or MTU. No significant change was observed in the fecal and urinary radioactivity after I131-T4 injection in thyroidectomized rats treated with mercaptoimidazole or potassium iodide.
2) The turnover rate of injected I131-T4 decreased in thyroidectomized rats treated with PTU or MTU.
3) Distribution of I131 in the liver and intestines (with contents) increased after 6 hours of I131-T4 injection in thyroidectomized rats treated with PTU.
4) The in vitro deiodination of I131-T4 in rat liver homogenate was significantly decreased by PTU or MTU administered in vivo.
5) A decrease in weight gain and an increase in the plasma PBI and I131-triiodothyronine (I131-T3) resin uptake was noted in T4-maintained thyroidectomized rats after the administration of PTU or MTU for 2 months.
6) When PTU-treated rats were administered with varying doses of T4 in order to prevent goiter formation, doses of T4 adequate to restore PBI to normal failed to prevent goiter completely, while goiter-preventing doses were associated with abormally high PBI values. Serum I131-T4 resin uptake was well related with PBI.
7) The turnover rate of injected I131-T4 decreased in euthyroid, hyperthyroid and hypothyroid patients by the administration of PTU or MTU.
8) Fecal excretion of I131 increased after I131-T4 injection in euthyroid patients administered with PTU or MTU. Urinary I131 decreased in one patient administered with PTU.
9) An increase in the PBI was observed after the administration of PTU for 4 weeks in athyrotic patients treated with desiccated thyroid. No significant change was observed in serun I131-T3 erythrocyte uptake in these patients.
These results indicate that PTU and MTU inhibit the peripheral metabolism of T4 and that the inhibitory effect may be independent on the the alterations of thyroxine binding capacity of serum proteins since no effect was observed on serum I131-T4 resin or erythrocyte uptake in these agents. It is suggested that the decrease in degradation of T4 caused by PTU might influence the secretion of TSH.
