Full Length Research Paper

Changes in the concentrations of thyroid hormones in freshwater catfish, Heteropneustes fossilis (Bloch), under exposure to sub-lethal concentrations of 2, 4-Dichlorophenoxyacetic acid (usually referred to as 2, 4 D, herbicide) and chlorpyrifos (organophosphate insecticide) were analyzed in this study. A comparison of the effects of 2, 4 D and chlorpyrifos at different concentrations on hormonal activities afforded a promising approach. The experiment was performed for 30 days and the fish were subjected to 1/10th and 1/50th Lethal Concentration (LC50) value of 2, 4 D (3.00 ppm and 0.60 ppm) and chlorpyrifos (1.42 ppm and 0.28 ppm). Both the xenobiotics were found to be effective in increasing the serum concentration of triiodothyronine (T3) and thyroxine (T4) up to the 20th day after which it showed a declining trend at the terminal phase of the experiment. This trend was observed to be dose dependent and time of exposure. The test chemicals exhibited drastic effect on thyroid stimulating hormone (TSH) level as indicated by significantly lower (p？0.01) concentration of TSH than that of the normal control group. In comparison to chlorpyrifos, 2, 4 D was observed to be more effective in decreasing the TSH level in the experimental fishes and in altering the hormone profile for the assessment of thyroid function