['Preparation with recombinant human thyroid-stimulating hormone (rhTSH) for thyroid remnant ablation results in lower extrathyroidal radiation than does hypothyroidism. The objective of this prospective study was to compare the damage caused by 131I (3.7 GBq) when these 2 preparations are used.', 'Ninety-four consecutive patients who underwent total thyroidectomy and remnant ablation with 3.7 GBq of 131I were studied. Thirty patients (group A) received rhTSH, and 64 (group B) were prepared by levothyroxine withdrawal. Damage to salivary glands, ovaries, and testes; hematologic damage; and oxidative injury were evaluated by measurement of serum amylase, follicle-stimulating hormone (FSH), complete blood count, and plasma 8-epi-PGF2alpha before and after radioiodine. The 2 groups were similar in sex, age, and the results of baseline assessment.', 'The rate of successful ablation (stimulated thyroglobulin level < 1 ng/mL and negative findings on neck ultrasonography) was 90% in group A and 80% in group B. Considering only patients with a preablation thyroglobulin level greater than 1 ng/mL, these rates were 80% and 70.6%, respectively. Only 1 patient (3.3%) reported transient headaches with rhTSH. Elevated FSH levels after therapy were observed in 4 of 9 (44%) men in group A versus 16 of 18 (89%) in group B (P < 0.03), with a mean increase of 105% versus 236% (P < 0.001), respectively. In women, elevated FSH was observed in 1 of 13 (7.7%) patients in group A versus 6 of 30 (20%) in group B (P = 0.4), with a mean increase of 65% versus 125% (P < 0.001). Thrombocytopenia or neutropenia occurred in 2 of 28 (7%) patients in group A versus 12 of 56 (21.4%) in group B (P = 0.1), with a mean decrease of 20% versus 45% and 25% versus 52% (P < 0.01) for neutrophils and platelets, respectively. Hyperamylasemia and symptoms of acute sialoadenitis occurred in 11 of 30 (36.6%) versus 48 of 60 (80%) (P < 0.001) and in 9 of 30 (30%) versus 35 of 60 (58.3%) (P = 0.01), respectively. 8-Epi-PGF2alpha was found to be elevated after 131I in 14 of 25 (56%) patients in group A versus 45 of 45 (100%) in group B (P < 0.001), with a mean increase of 60% versus 125% (P < 0.001).'] Is preparation with recombinant human thyroid-stimulating hormone for thyroid remnant ablation with 131I associated with lowered radiotoxicity?