OBJECTIVE To determine the value of the use of radio-iodine scanning diagnostic and radio-iodine therapy during the follow-up of advanced differentiated thyroid cancer without stopping thyroid hormone suppression therapy. METHODS We performed 7 radio-iodine scans and 4 radioiodine therapies in 5 men and 1 woman, aged 39 to 79 years. Five patients had papillary thyroid cancer and one follicular thyroid cancer. Human recombinant thyrotropin was given intramuscularly at a dose of 0.9 mg/ml once a day for two days. Twenty-four hours after the second dose, serum thyrotropin, thyroglobulin (Tg) and thyroglobulin autoantibodies were measured and each patient was given 2 mCi of 131I in postsurgery scans and 5 mCi of 131I in patients previously treated with 131I. Whole-body scans were obtained within 54 hours of injection and in the cases of diagnostic uncertainty a second scan was performed 72 hours after the injection. RESULTS Four scans were positive and two were negative. One positive scan patient was not treated due to a serious clinical condition. An effective increase of serum TSH was observed in all 6 patients. Four patients had an effective increase of serum Tg; the other 2 cases had a decrease of serum Tg: One case was undifferentiated and in the other case the mass was smaller after surgery. CONCLUSIONS rhTSH is an effective means of stimulating TSH levels and allowing scan and/or radioiodine treatment. rhTSH has no significant adverse effects. The use of rhTSH avoids all the disadvantages of hypothyroidism and maintains quality of life.