Toxicology 195 (2004) 69–75 Opposite effects of zinc and melatonin on thyroid hormones in rats Abdulkerim Kasim Baltaci a,∗ , Rasim Mogulkoc a , Aylin Kul a , Cem Seref Bediz b , Aysegul Ugur c a Department of Physiology, Meram Medical School, Selcuk University, Konya 42080, Turkey b Department of Physiology, Medical School, Dokuz Eylul University, Izmir, Turkey c Meram Medical School, Selcuk University, Konya 42080, Turkey Received 2 June 2003; received in revised form 1 September 2003; accepted 1 September 2003 Abstract The present study was conducted to investigate how thyroid function in rats is affected by administration of 3mg per kg per day of zinc and/or melatonin. The study was conducted with 40 Sprague–Dawley adult male rats equally divided into four groups: 1 (controls), 2 (zinc-only), 3 (melatonin-only) and 4 (zinc- and melatonin-supplemented). The supplementation was continued for 4 weeks after which the animals were sacrificed and plasma samples were obtained for determination of zinc, melatonin, free- and total triiodothyronine (T3), thyroxine (T4) and thyroid-stimulating hormone (TSH) levels. The free T3, T4 and TSH levels were lower in the melatonin group than in all other groups (P< 0.01), while free- and total T3 levels were higher in the zinc group (P< 0.01). The group that received zinc and melatonin combined had free thyroid hormone levels higher than the only melatonin group. These results show that melatonin has a thyroid function suppressing action, just the opposite to the actions of zinc. When zinc is administered along with melatonin, its thyroid function suppression is diminished. © 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Zinc; Melatonin; Thyroid hormone function; Rats 1. Introduction Melatonin (MT) is a hormone secreted by the pineal gland. It has been found to have a general inhibitor ef- fect on thyroid hormones (Lewinski, 2002; Mogulkoc and Baltaci, 2002; Wright et al., 1996). Melatonin in- jections led to a reduction in blood TSH and thyroid ∗ Corresponding author. E-mail address: baltaci@selcuk.edu.tr (A.K. Baltaci). hormone concentrations in rats (Ozturk et al., 2000), although lack of such an effect has also been reported (Ashley et al., 1999). The effects of MT on thyroid hor- mone secretion may be either direct or indirect and the corresponding mechanisms are still to be established. Preliminary studies on this subject showed that MT administration increased stimulating effect of TSH on thyroxin secretion (Csaba and Nagy, 1976). It was also seen that long-term MT administration in male rats in- creased TSH, but did not make a change in peripheral 0300-483X/$ – see front matter © 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.tox.2003.09.001