The Minimum Dosage of Thiourea Effectively Inhibiting Thyroxine Synthesis in the Flounder (Platichthys Stellatus)

Thiourea in concentrations of 0.0025%, 0.005%, 0.01%, 0.02%, and 0.03% prevented the synthesis of thyroxine by the thyroid gland of the starry flounder (Platichthys stellatus). Concentrations of 0.001%, 0.0005%, and 0.0001% thiourea were not inhibitory. Concentrations of 0.005%–0.03% thiourea were effective for over 9 weeks. Flounder exposed to thiourea retained relatively little I131 in their thyroids. Although the synthesis of thyroxine was completely blocked by the inhibiting concentrations of thiourea, some synthesis of monoiodotyrosine and diiodotyrosine occurred.