Differentiated thyroid cancer, like breast cancer, prostatic cancer, and endometrial cancer of the uterus, is well known to be hormone sensitive. Experimental investigations have demonstrated that differentiated thyroid cancer cells have TSH (thyroid-stimulating hormone) receptor on the plasma membrane and that the growth regulation of differentiated thyroid cancer depends upon TSH. Therefore, suppression of TSH with thyroid hormone is rational for the treatment of recurrent thyroid cancer. Recurrent differentiated thyroid cancers reportedly cause regression in response to thyroid hormone administration, but the outcome of adjuvant therapy with thyroid hormone after operation for differentiated thyroid carcinoma is controversial. It is very difficult to analyze the difference in survival rate between the postoperative patient with and without thyroid hormone, because of the excellent postoperative survival rate of differentiated thyroid cancer patients. Further clinical studies and laboratory investigations about TSH suppression in adjuvant therapy for differentiated thyroid cancer are necessary to elucidate the impact of thyroid hormone on survival after operation.