Toxic effects of mercurials on thyroid function of the catfish, Clarias batrachus (L.)

The toxic effects of mercurials on the thyroid activity of Clarias batrachus were evaluated after exposure for 45, 90, and 180 days during gonadal recrudescence. Histologically, the thyroid follicles were active with hypertrophied epithelium and reduced colloid content in 90- and 180-day mercuric chloride (HgCl2)- and 90-day methylmercuric chloride (CH3HgCl)-treated fish. In 180-day CH3HgCl and 90- and 180-day emisan 6 groups, the follicular epithelium appeared atrophic with a wavy and irregular outline and with hypertrophied and hyperplastic cells present in some parts of the epithelium. Thyroidal 131I uptake was inhibited in all groups and at all durations. Protein-bound iodine (PBI) levels decreased significantly in the 45-day CH3HgCl group and in all the mercurial groups treated for 90 and 180 days. Of the three mercurials, CH3HgCl was found to be more toxic than the other two in inhibiting thyroidal 131I uptake and PBI level at all durations.