Differentiated thyroid carcinoma (DTC) is one of malignant neoplasms with a very good prognosis and low mortality rate provided a proper therapy and its systematic monitoring is given. DTC treatment consists of surgery, radioiodine and L-thyroxine therapy. L-thyroxine therapy in DTC should be both substitutive and suppressive. Substitutive therapy consists in the removal of the lack of thyroid hormones symptoms so that the serum TSH is about 1 mU/l. Suppressive therapy consists in complete inhibition of TSH secretion by pituitary gland. This prevents from the regrowth and inhibition of carcinoma progress in patients with evident or residual neoplasmic disease. Although complete suppressive therapy (TSH less than 0.05 mU/l) is the most recommended one, in patients with low recurrence risk incomplete suppression can be used (TSH = 0.1-0.3 mU/l). Suppressive L-thyroxine doses may give side-effects such as: cardiac signs (arrhythmia, coronary or heart insufficiency), decrease in bone mineral density with osteopoenia or osteoporosis or subclinical hyperthyroidism. However, the side-effects of the suppressive L-thyroxine doses may be symptomatically treated (with beta-blockers, biphosphoniates).