Thyroid hormone plays a vital role in regulating differentiation, growth, cognitive development, metabolism and physiological functions of virtually all tissues of vertebrates. Mammalian testis is a target of thyroid hormone action and altered thyroid status is well known to modify testicular functions. Hyper-metabolic state resulted in hyperthyroidismconsequences in increase in free radical production. As, testis is very rich in polyunsaturated fatty acids with poor antioxidant defense system, it is much more vulnerable to oxidative damage in compared to other tissues. Altered testicular antioxidant defence system by hyper- and hypothyroidism keeps testis under oxidative stress in consequence influencing its physiology.