Background & Objectives: Zinc as a heavy metal is known to be an environment pollutant. Research shows that high amount of this metal might pose health hazards by affecting the normal function of endocrine glands and metabolic activities. Thus, this study was conducted with the aim of determining the effect of various amounts of zinc on thyroid gland and liver enzymes in adult male rats in Kazeroon in 2005.
Materials & Methods: In this experimental study 40 adult male wistar rats were classified in to experimental, control and sham groups. Experimental groups received zinc in from of oral zinc sulfate 20 , 40 and 80 mg/kg for 15 days in three subgroups . Sham group received 1 ml distilled water while control group received no substance. At the end of 15th day blood sample was taken from ventricular part of the heart and concentrations of T3, T4 and TSH hormones and AST, ALT and ALP enzymes were measured through ELISA and trade kits and the data were analysed using Tukey test. 
Results: The results showed no significant difference in T3 hormone concentration and ALP enzyme between experimental groups and control group, while T4 hormone concentration in the first experimental group (20mg/kg zinc sulfate) increased significantly compared with control group (P<0.05). The concentration of TSH hormone showed a significant decrease between experimental and control groups (P<0.05). Serum concentration of AST and ALT in the first experimental group increased significantly compared to control group while it showed a significant decrease in the second and third experimental groups (40 and 80 mg/kg zinc sulfate) (P=0.05).
Conclusion: The study results indicate that zinc affects thyroid gland and blocks the activity of 5'- diodinase through binding to sulfhydryl groups and decreases the conversion of T4 to T3. Zinc also decreases TSH amount and can damage plasma membrane and increase the amount of AST and ALT enzyme.