Cythion-induced histophysiological changes in thyroid and thyrotrophs of the teleost fish, Channa punctatus (Bloch)

The chronic effects of continuous exposure for 6 months, from January to June, to safe (2.0 ppm) and sublethal (4.0 ppm) concentrations of a commonly used organophosphorus pesticide, cythion, on the histophysiology of the thyroid and thyrotrophs in Channa punctatus are described in this investigation. The pharyngeal thyroid of fish of both treated groups exhibited hypertrophy and hyperplasia of the follicular epithelium and reduction of colloid content. Apart from this, in the fish exposed to 4.0 ppm, some of the follicles were exhausted and transformed into degenerated cystic masses. The presence of free blood cells at these sites suggests rupture of blood vessels and possible phagocytosis. These histopathological changes could be correlated with a significant and dose-dependent reduction of thyroidal radioiodine uptake and conversion ratio (serum protein-bound iodine:total serum iodine) values. After cythion exposure, the thyrotrophs in the pituitary were activated. These results suggest that the thyroid dysfunction caused by cythion may be responsible for the stimulation of the thyrotrophs which in turn stimulated the thyroid follicles through the pituitary-thyroid axis in this species.