Comparison of 1073 MBq and 3700 MBq iodine-131 in postoperative ablation of residual thyroid tissue in patients with differentiated thyroid cancer. In a randomized prospective study, we compared the efficacy of low dose (1073 MBq) and high dose (3700 MBq) iodine-131 administration in postoperative ablation of residual functioning thyroid tissue in 63 patients with differentiated thyroid cancer. We were unable to demonstrate any difference between the low- and the high-dose of radioactive iodine in scintigraphic ablation of remnant tissue. In 81% (21/26) of the patients, 1073 MBq ablated after the first dose, 77% (21/26 + 3/5 = 24/31) after the first plus second dose, and 69% (24/31 + 0/4 = 24/35) after the first, second, and third dose. Radioiodine (3700 MBq) ablated in 84%, 73%, and 69% of the patients after respectively 1., 1. plus 2., and 1. plus 2. plus 3. dose. Forty percent of the patients ablated with the low dose and 44% ablated with the high dose had elevated thyroglobulin levels at the time of complete scintigraphic ablation. In conclusion, we did not find any difference between 3700 MBq and 1073 MBq iodine-131 as regard to number of doses needed for complete scintigraphic ablation of residual functioning thyroid tissue. 