The study will enroll approximately 90 patients. Patients receiving the same daily dose of L-Thyroxine Christiaens® during the past 6 weeks and with serum thyroid stimulating hormone (TSH) levels between 0.4-2.5 mU/L will be switched to the equivalent daily dose of the new formulation of levothyroxine. After the switch, patients will be followed up after 2 (± 2 weeks) and 4 months (± 4 weeks).