Synthetic Glucocorticosteroids (GCs) are widely used in veterinary medicine for their anti-inflammatory and immunosuppressive actions. However they are deemed to have multiple side effects such as hypothyroidism. The aim of this study was to determinate the effects of Dexamethasone (DEX) administered parenterally (IM) at therapeutic doses, conventionally used in the anti-inflammatory and immunosuppressive treatments on the thyroid function of sheep. Fifteen sheep of Ouled Djellal breeds were randomly divided into three groups of 5 rams. The animals received an injection of DEX (DEXALONEND solution) at a rate of 1.52 mg DEX/d for group 1 and 3.4 mg DEX/d for group 2 respectively, during 6 days; the third group has served as a control group and did not receive any treatment. Blood samples taken over a period of 3 weeks were analyzed to define the plasma levels of free thyroid hormones fT4 and fT3. It has been shown that Dexamethasone induced a significant decrease (P < 0.05) of serum fT4 and fT3 24 h following the beginning of the treatment. Hormone levels have returned to their baseline values 24 h after treatment withdrawal. From these results, we can deduce that DEX antithyroid effects are transitory and therefore do not have a significant impact on the health of the animals.