Effect of pesticide on circulating thyroid hormone levels in the freshwater catfish, Heteropneustes fossilis (Bloch).

The effects of malathion (an organophosphorus) and BHC (an organochlorine) exposure on circulating levels of plasma T3 and T4 were studied in the freshwater catfish, Heteropneustes fossilis. A lowered plasma T4 level was noticed within 4 weeks of malathion (10 and 20 ppm) and BHC (8 ppm) exposure. Higher concentrations of BHC (16 ppm) induced an increase in the plasma T4 level. BHC-exposed fish exhibited significantly lowered plasma T3 and T3/T4 ratio, but malathion exposure significantly increased their levels. Probably BHC acted directly on the thyroid gland, reducing the synthesis and release of the thyroid hormone. Malathion seems to increase the plasma T3 level either by stimulating the extrathyroidal conversion of T4 to T3 or by reducing T3 excretion, thereby increasing plasma T3 and the T3/T4 ratio in this fish.