['Recombinant human thyroid-stimulating hormone (rhTSH) is effectively used for exogenous thyroid-stimulating hormone (TSH) stimulation before diagnostic (131)I scintigraphy. It is not yet widely used for preparation of patients receiving a therapeutic amount of radioiodine.', 'The results of 64 consecutive therapeutic applications of rhTSH with regard to clinical tolerance and side effects were evaluated in comparison with 163 radioiodine therapies (RITs) done on patients with hypothyroidism after thyroxine withdrawal during the same period. All therapies-applying 1.1-10 GBq of (131)I-used a standardized protocol of patient preparation and activity application. RITs were followed by daily whole-body uptake measurements for 2-6 d, and a biexponential curve fit was used to obtain a short initial and afterward a long effective half-life of (131)I. Patients after rhTSH were evaluated as a whole group (group A, n = 64) and as a subset of that group with normal thyroglobulin (hTG) levels (group D, n = 18). Patients after endogenous TSH stimulation were evaluated as a whole group (group B, n = 163), as a subset of that group excluding all ablative RITs (group C, n = 113), and as a subset of that subset with normal hTG levels (group E, n = 87).', 'rhTSH-stimulated patients showed significantly higher TSH values than did endogenously stimulated patients (P < 0.001). Furthermore, the effective half-life of (131)I was significantly prolonged after endogenous stimulation (e.g., 0.43 d for group A vs. 0. 54 d for group B, P < 0.001). All rhTSH applications were tolerated well and without serious side effects. The only side effects were 2 cases of nausea and headache.'] Does rhTSH stimulation before radioiodine therapy in thyroid cancer reduce the effective half-life of ( 131 ) I?