Abstract. Earlier studies of the effect of glucocorticoids on thyroid function tests have given different results. This may be due to the fact that varying corticosteroid doses have been used. The present study examines four groups of healthy volunteers, who were given betamethasone in increasing doses (1.5, 3.0, 4.5 and 6.0 mg daily) for 5 days. Definite dose-response relations were found for serum T4 and T3 while no clear-cut dose-response pattern was found for rT3. The results obtained explain previous variable results. Oral corticosteroids also affected the concentrations of the thyroid hormone-binding proteins but no clear-cut dose-response relation were observed.