Effect of graded doses of propylthiouracil on in vitro iodoamino acid synthesis in the rat thyroid gland

The effect of graded concentrations of propylthiouracil on the synthesis of labeled iodoamino acids was studied under in vitro conditions in which rat thyroid lobes are capable of synthesizing significant quantities of radioactive iodothyronines. The ease with which synthesis of the iodoamino acids was inhibited by PTU was in the following order: T3 and T4>DIT>MIT. Since the facility with which formation of each labeled substance was depressed by PTU was inversely related to its magnitude of formation in the absence of PTU, no definite conclusions as to relative susceptibility of the various biosynthetic steps to inhibition by PTU are possible. Nevertheless, the pattern of inhibition is similar to previous in vivo data. PTU enters the thyroid lobes from the medium rapidly enough to prevent organic binding of inorganic iodide present in the lobes before incubation. Apparently, iodide present in the inorganic form in the thyroid for long periods can be organically bound as rapidly as that newly entering the...