Background: Studies from every continent have shown that only around 50% of the patients subjected to thyroid hormone replacement have TSH in the normal range. However, to date, there are no consistent data about Brazil. Objectives: To evaluate levothyroxine (LT4) replacement treatment in patients with primary hypothyroidism followed in referral centers in Brazil. Methods: Patients with primary hypothyroidism followed in referral centers (University Hospitals from Universidade Federal do Rio de Janeiro - UFRJ, Unicamp, Universidade Federal do Parana - UFPR and Universidade Federal do Ceara-UFC) answered a questionnaire that inquired about clinical and biochemical conditions, social-economic status, life quality and clinicians' orientations as well as their understanding about the information given. Serum TSH was checked close to the interview. Results: 2292 consecutive patients met the inclusion criteria. Mean age 51.2 yr and TSH values between 0.4 and 4.0 mUl/l were considered to be within the reference range. Among all patients taking thyroid medication, 42.7% had an abnormal serum TSH (28.3% were undertreated and 14.4% were overtreated). Approximately all patients (99%) took LT4 in the morning but less than 30 min before breakfast (85.4%). Regarding the clinicians' orientations: 97.5% of the patients were instructed to take the medication daily, and 92.6% to take 30 min before breakfast (92.6%). However, only 52.1% were told not to take LT4 along with other medication. Conclusions: Our study found that a significant number of patients taking thyroid hormones were not in the therapeutic range. Clinicians should, therefore, consider monitoring patients on thyroid replacement more frequently and being more precise on giving recommendations about the correct use of LT4. (C)2013, Editrice Kurtis367485488MERCK SERONO