To investigate the effect of the combination of dexamethasone (Dex) and sodium ipodate (SI) on hyperthyroidism, we studied 24 patients with typical Graves' disease, divided into four groups of six persons each. Three groups (Study I) were studied acutely (24 h) to determine the effects of Dex (5 mg every 12 h intramuscularly), SI (one oral dose of 3 g) and both drugs at the same doses, upon T4, T3, and rT3 at 0900 h before therapy was started and 24 h later. The group on Dex and that on SI had a similar T3 decrement of 25.9 +/- 4.0% and 35.8 +/- 5.0%, respectively, (P less than 0.05), whereas the effect of both drugs combined was greater (64.2 +/- 3.6%; P less than 0.01, Dex, and P less than 0.01, SI, respectively). The increment of rT3 was markedly greater in those patients on SI than in those on Dex (561.3 +/- 149.2% and 58.9 +/- 11%, respectively, P less than 0.025). A fourth group (Study II) was studied for seven days while receiving both Dex (1 mg orally three times per day) and SI (500 mg orally three times per day). Both T4 (from 18.8 +/- 1.1 to 13.1 +/- 1.1 micrograms/dl, P less than 0.02) and T3 (from 593 +/- 41 to 136.3 +/- 12.7 ng/dl, P less than 0.001) decreased at day 8. The initial brisk increment of rT3 at 24 h (808 +/- 149%, P less than 0.005) then diminished concomitantly with the fall of its precursor, T4.(ABSTRACT TRUNCATED AT 250 WORDS)