Introduction: Thyroid hormones play an important role in cell growth and differentiation and regulation of oxygen consumption and thermogenesis. The effect of altitude and vitamin supplementation on thyroid hormone levels in animals or humans performing acute exhaustive exercise have not been investigated before. Therefore, we sought to test whether exhaustive exercise-induced stress with antioxidant supplementation was capable of modulating the level of thyroid hormones at different altitudes. Methods: In both low and high altitude areas, native untrained rats were divided equally into three groups, each (N= 6); control group (non-stressed and untreated); stress group A (received normal saline); and stress group B, received a single intra-peritoneal dose of 25 mg/kg of vitamin E in combination to 20 mg/kg of Vitamin C orally for one hour before the beginning of the experimental procedure. Stress was achieved in groups A and B by forcing the rats to swim for a duration of 2.5 h in glass tanks (length 100cm, width 40 cm, depth 60 cm) containing tap water maintained at a temperature of 32 o C. The depth of water in the tank was 30 cm. At the end of the experimental procedure, rats were terminally anesthetized with light diethyl ether and blood samples were taken at the same time directly from the heart and used to measure serum levels of T3, T4 and TSH. Results: Thyroid levels were significantly decreased in resting rats at HA compared to LA, and swimming exercise moderately increased T3 and TSH at both LA and HA, whereas T4 was markedly increased (62%) at LA compared to a moderate HA increase (28%). Co-administration of vitamins C and E augmented the observed forced swimming-induced thyroid release. However, the conversion of T4 to T3 was reduced in both altitude areas following swimming exercises and vitamin pre-treatment had no effect. Conclusions: We conclude that acute stress induced thyroidal hormones in rats was augmented by antioxidant drugs in both LA and HA areas. These findings may play an important role in the human pathophysiology of thyroid gland at different altitudes.