الفهرس 
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Abstract
The present study was carried out to investigate the effects of melatonin on the reproductive disorder and organ damage induced by hyperthyroidism. A total 75 male rats were divided into five groups 15 rats each: control group (distilled water containing 4M ammonium hydroxide in methanol and 1% absolute ethanol), L-thyroxine group (0.2 mg/kg), L-thyroxine plus melatonin (1, 5 and 10 mg/ kg), L-thyroxine and melatonin were injected intraperitoneally daily for 21 days. Fasting blood samples were collected from each rat early in the morning at 11th day and at the end of the experiment, serum was separated, fasting blood glucose level was measured immediately after serum collection and the serum stored either at -20 ˚C to be used for determination of liver function tests (AST, ALT and ALP), kidney function tests (creatinine level and urea concentration), lipogram (total cholesterol, triglycerides, HDL and LDL) and hormones (T4, T3, TSH, FSH, LH and testosterone) or at -80 ˚C for determination of total antioxidants capacity. Five rats were scarified from each group in the 11th and at the end of the experiment and the following organs were collected to be used as follow Liver, kidney and heart were used for estimation of oxidant and antioxidant parameters (MDA, SOD and GSH), Liver and kidney were used for DNA damage and Liver, kidney, heart and testis were used for histopathological examination at the end of experiment. The results revealed that hyperthyroidism increased the levels of glucose, urea, liver enzymes activities, T3, T4, testosterone, percentage of DNA fragmentation and MDA concentration with histopathological alteration. At the same time hyperthyroidism decreased the performance traits, creatinine, lipogram, TSH, LH, FSH, total antioxidants capacity, SOD and GSH activity. Melatonin hormone could counteract the harmful effects of hyperthyroidism on the aforementioned parameters but couldn’t ameliorate the reproductive disorders.