Cardioselective [Acebutolol (N = 10)] or noncardioselective [Oxprenolol (N = 9)] or [Pindolol (N = 9)] beta-adrenergic blockers were given to patients with suspected hyperthyroidism. Four h after start of the betablockade median serum thyroglobulin (Tg) had decreased to 33 (range: 13–325) μg/l from 41 (range: 12–333) μg/l before start in the acebutolol treated group (p < 0.05). A significant decrease in serum Tg was also found in the oxprenolol-treated group [before start: 45 (24–423) μg/l and after 4 h: 43 (18–363) μg/l (p < 0.01)] and in the pindolol-treated group [before start: 154 (33–210) μg/l and after 4 h: 63 (19–157) μg/l (p<0.05) treated groups. After 7 days treatment the decrease in serum Tg was significant [to 85 (34–182) μg/l (p< 0.02)] only in the Pindolol-treated group. It is suggested that both hemodynamic changes as well as interference with the intrathyroidal 12 S Tg to 19 S Tg dimerization might explain the changes in serum Tg during betablockade. Knowledge of drug influence on serum thyroglobulin is important for the interpretation of variations found in patients where thyroglobulin is being used for diagnostic purposes.