Thyroid physiology impairment by malathion in the freshwater catfish Clarias batrachus.

Effects of malathion on serum and glandular thyroxine (T4), triiodothyronine (T3), the T3/T4 ratio, peroxidase activity, and extrathyroidal conversion of T4 to T3 have been studied in the freshwater catfish Clarias batrachus during the prespawning and spawning phases of its annual reproductive cycle. Malathion inhibited T4 secretion in the kidney, but accelerated T4 synthesis in the pharyngeal thyroid. This pesticide also inhibited the extrathyroidal conversion of T4 to T3 in serum during both the studied phases.