BACKGROUND: Glucocorticoids are the steroidal drugs which are very widely used in large animal medicine. These agents have advantages in large animals but they have been also associated with many potential adverse effects especially at high doses or prolonged use. OBJECTIVES: The present experimental study was designed to clarify the effects of dexamethasone (DEXA) and isoflupredone (ISO), as the most common glucocorticoids in large animal medicine, on bovine thyroid hormones. METHODS: Ten clinically healthy Holstein calves (6-8 months old) were assigned into 2 equal groups. Dexamethasone (1 mg/kg) and isoflupredone (1 mg/kg) were administered intramuscularly in DEXA and ISO groups, respectively, for two consecutive days. Blood samples were taken at days 0 (before the 1st dose), 1 (before the 2nd dose), 2, 3, 5 and 7, from all studied animals and serum concentrations of T3, T4, fT3 and fT4 were determined in all specimens. RESULTS: Levels of T3 and T4 were decreased significantly after both drugs administrations. The concentrations of T3 and T4 in Iso group were significantly lower than DEXA one (P<0.05). There were no significant changes in serum fT3 and fT4 levels following drugs administrations. CONCLUSIONS: Pharmacological doses of dexamethasone and isoflupredone have suppressive actions on the circulating levels of thyroid hormones in Holstein calves possibly via inhibition of TSH production at hypothalamic-pituitary-thyroid level.