Objective: To establish limits of normal serum TSH for the adult (18 to 60 years) Brazilian population according to recommendations of the National Academy of Clinical Biochemistry.
Subjects and methods: Healthy volunteers were evaluated and those fulfilling the following clinical criteria were selected: absence of known thyroid disease; no use of any interfering medications; no history of head and neck external radiotherapy, type 1 diabetes or autoimmune disease; no family history of thyroid disease, and absence of goiter or palpable nodules. Subjects with anti-thyroperoxidase antibodies and/or altered free T4 were excluded. The sample consisted of 960 subjects (480 males and 480 females).
Results: TSH values corresponding to the 2.5th and 97.5th percentiles of the sample were 0.43 and 3.24 mIU/L, respectively. TSH values > 2.5 mIU/L were observed in 9.15% of the volunteers and levels > 3 mIU/L in 3.11%.
Conclusion: The present study suggests an upper limit of normal TSH of approximately 3.5 mIU/L.