The thyroid gland and its hormones have been reported to influence reproduction and metabolism in a positive manner. However, research to date provides strong evidence for a reciprocal relationship between the immune system and hormones of the hypothalamus-pituitary-thyroid axis. The present study has been taken to elucidate the effect of L-thyroxin (T4) on immune parameters such as total leukocyte count, percent lymphocyte count, blastogenic response and percent stimulation ratio of thymocytes and splenocytes of a seasonally breeding rodent, Funambulus pennanti, during its reproductively inactive phase when peripheral T4 is low. Treatment with L-thyroxin at a near-physiological dose of 35 µg/100 g body weight during evening hours showed a significant elevation of all immune parameters. However, combined treatment of L-thyroxin and melatonin (25 µg/100 g body weight) had no additive effect. On the other hand, in vitro supplementation of T4 (10–6M) either alone or in combination with melatonin (0.5 µM) did not induce any significant change on thymocyte and splenocyte proliferation. Therefore, a trade-off effect of L-thyroxin and melatonin on the immune system (T- and B-cell differentiation) is suggested for this rodent.

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