['Strategies to increase frequency of euthyroidism following radioactive (RAI) treatment of hyperthyroidism are required.', "To examine the role of TSH in development of hypothyroidism post RAI treatment in patients with Graves' disease (N = 98) or toxic nodular goiter, TNG (N = 88).", 'This retrospective study examined thyroid status over a mean of 3.7 years post-RAI.', "Although RAI dose was significantly higher in TNG group, hypothyroidism occurred more frequently in Graves' disease (71.4 and 22.7%) P < 0.001. The TSH levels at the time of RAI treatment were lower in TNG patients who remained euthyroid, (0.4+/-0.1 vs. 1.2+/-0.5 mU/l, P < 0.0022)."] Is the occurrence of hypothyroidism following radioactive iodine treatment of toxic nodular goiter related to the TSH level?