Background: Decreased muscle strength and exercise intolerance are prominent features of hyperthyroid patients. Skeletal muscle dysfunction may be severe in newly diagnosed hyperthyroid patients, compromising their abilities to perform daily activities. Thyroid hormones have shown to affect mitochondrial oxidative activity, synthesis and degradation of protein, differentiation of muscle fibers and capillary growth. Objective:  Studying the effects of experimentally-induced hyperthyroidism on acute exercise tolerance in male albino rats. Material and methods: Thirty adult male albino rats of local strain were chosen to be the model of the present study. They were divided into three equal groups;control group, hyperthyroid group (subjected to induction of hyperthyroidism by daily IP injection of 10 μg thyroxin /100 gm body weightfor four weeks),and recovery group. Rats were weighed at the start and the end of the work. Blood samples were obtained for determination of serum free T₃, free T₄, TSH, CPK, TAC and MDA levels. Results: Induction of hyperthyroidism led to significant decrease in the body weight, swimming time, TSH level and total antioxidant capacity associated with significant increase in the FT₃, FT₄, CPK and MDA levels. These changes improved by allowing recovery of hyperthyroid state by thyroxin withdrawal over time. However, these improvements did not resume the basal level. Conclusion: Hyperthyroidism has a drawback effects on the body functions. These effects were improved by withdrawal of thyroxin (or treatment) without resuming the basal level of  euthyroid state. Further studies are required to evaluate the effects of prolonged time of recovery or treatment of hyperthyroidism on resuming basal levels of disturbed body functions.