Biochemical Effects of Levothyroxine Withdrawal in Patients with Differentiated Thyroid Cancer. 
Patients with differentiated thyroid carcinoma (DTC) are submitted to withdrawal of levothyroxine (LT4) aftter thyroidectomy, in order to undergo radiodine ((131)I) treatment.  A total of 345 patients with a history of DTC were enrolled in the study.  Their biochemical profile and serum free triiodothyronine (FT3), free thyroxine (FT4) and thyrotropin (TSH) levels were measured during withdrawal of LT4 treatment, and several months after restarting LT4.  During withdrawal, the intra-individual percentage increase in total cholesterol, low density lipoprotein-cholesterol, very low density lipoprotein-cholesterol and triglycerides was of the order of 60-80% and that for high density lipoprotein-cholesterol 30%.  Creatinine increased by 30%, whereas Na and K levels decreased by 1%.  The increase for creatine phosphate kinase was around 200-300%, for aspartate aminotransferase and alanine aminotransferase 50-80%, for γ-glutamyl transpeptidase 10-20%, and for lactate dehydrogenase 25%.  Glucose decreased by 1-4%.  Short-term, acute hypothyroidism in patients with DTC induces significant alterations in several biochemical parameters.  The presence of other deteriorating diseases should be considered before submitting these patients to LT4 withdrawal.