To evaluate the impact of genetic polymorphisms in uridine 5'-glucuronosylytansferases UGT1A1 and UGT1A3 and iodothyronine-deiodinases types 1 and 2 on levothyroxine (T4 ; 3,5,3',5'-triiodo-L-thyronine) dose requirement for suppression of thyrotropin (TSH) secretion in patients with differentiated thyroid cancer (DTC).