Recurrence of Differentiated Thyroid Carcinoma During Full TSH Suppression: Is the Tumor Now Thyroid Hormone Dependent?

Well-standardized primary treatment and long-term management of differentiated thyroid carcinoma (DTC) include lowering or suppression of host thyrotropin (TSH) with exogenous L-thyroxine (T(4)). This treatment recognizes the trophic action of TSH on DTC cells. Suppression of endogenous TSH with T(4) is continued in recurrent disease. However, T(4) can induce proliferation of follicular and papillary thyroid carcinoma cell lines and of other human carcinoma cells. The proliferative mechanism is initiated at a cell surface receptor for T(4) on integrin αvβ3, a receptor by which the hormone also inhibits p53-dependent apoptosis in tumor cells. In recurrent DTC with satisfactory suppression of endogenous TSH, we discuss here the possibility that the tumor is no longer TSH dependent and that T(4) has become a critical growth factor for the cancer.