The present study was conducted to investigate how pinealectomy and zinc deficiency separately or in combination affected thyroid hormones in rats. The study was carried out on 40 Sprague-Dawley male rats. The rats were equally allocated to four groups: Group 1 (control group), Group 2 (zinc-deficient group), Group 3 (pinealectomized group) and Group 4 (pinealectomized and zinc-deficient group). At the end of a 4-week study period, the rats were decapitated and blood samples were taken. The samples were examined in terms of plasma zinc, melatonin, free and total T-3, T-4, and TSH. It was found that free T-3 and T-4 levels in the pinealectomized group (Group 3) were higher than all others (p < 0.01) while free T-3, T-4, and TSH levels in the zinc-deficient group (Group 2) were lower than all others (p < 0.01). Free T3 and T4 levels in the pinealectomized zinc-deficient group (Group 4) were lower than those in Groups I and 3 and higher than those in Group 2 (p < 0.01). The findings obtained at the end of the study period show that pinealectomy has a stimulating and zinc deficiency has a suppressing effect on thyroid hormones and that the suppressing effect caused by zinc deficiency is partially balanced by pinealectomy.
