Carbaryl-induced thyroid dysfunction in the freshwater catfish Clarias batrachus.

Freshwater female catfish, Clarias batrachus, were exposed to sublethal concentrations of carbaryl (12 mg/liter for 96 hr and 5 mg/liter for 16 days) during the prespawning and spawning phases of their annual reproductive cycle. The effects on the levels of thyroxine (T4) and triiodothyronine (T3), the T3/T4 ratio, and the extrathyroidal conversion of T4 to T3 were studied in serum, pharyngeal thyroid, and anterior and posterior kidney. Carbaryl exposure for 96 hr suppressed T4 but elevated T3 and the T3/T4 ratio in serum during both phases of experimentation. However, 16 days exposure caused a decline in T4 as well as in T3 and in the T3/T4 ratio. In the pharyngeal thyroid, carbaryl increased T4 as well as peroxidase activity and decreased T3 and the T3/T4 ratio. Its treatment for 96 hr did not elicit any change in the parameters studied in the anterior kidney, whereas in the posterior kidney it suppressed the levels of T4 and T3, peroxidase activity, and the T3/T4 ratio. However, carbaryl exposure for 16 days provoked increases in T4 and peroxidase activity but decreases in T3 and the T3/T4 ratio in anterior kidney. In posterior kidney, it did not change T4 and peroxidase activity, but decreased T3 and the T3/T4 ratio. It also stimulated extrathyroidal conversion of T4 to T3.