Radioiodine has been shown to reduce recurrences and improve survival in well-differentiated thyroid cancer. To maximize the effectiveness of radioiodine therapy, patients are first treated by total thyroidectomy and then allowed to become hypothyroid. The elevation of thyroid-stimulating hormone, or thyrotropin (TSH), that occurs with hypothyroidism stimulates uptake of radioiodine in normal and cancerous thyroid tissues. A recent advance has been the introduction of recombinant human TSH (rhTSH), which is administered intramuscularly prior to testing with radioiodine. Phase III trials have demonstrated that rhTSH stimulates both uptake in and production of thyroglobulin by thyroid cells and the results are comparable to those of hypothyroid protocols in the majority of patients. Patients prefer the rhTSH protocol because they continue to ingest exogenous thyroid hormone and the symptoms of hypothyroidism are avoided. The rhTSH protocol is preferable in patients with pituitary dysfunction and in those who cannot tolerate hypothyroidism. RhTSH can also allow treatment of patients who have not had an adequate thyroidectomy and who are poor candidates for reoperation.