Regional adjustment of thyroid hormone reference intervals

Abstract

Background: We conducted a study in a metropolitan area to establish regional reference intervals for thyreotropin (TSH) and the thyroid hormones free triiodthyronine (fT3) and free thyroxine (fT4). This was due to the different reference ranges, based on varying regional trials, presented for a widely used electrochemiluminescence immunoassay system. Methods: We investigated 292 apparently healthy adult subjects and excluded those with known history of thyroid disease, abnormal findings in the ultrasonographic examination of the thyroid gland, or elevated thyroid autoantibodies in serum. Accordingly, 204 of 292 subjects were included as the reference collective. We measured serum concentrations of TSH, fT3, and fT4 using the Elecsys assays from Roche Diagnostics and calculated the 2.5th and 97.5th percentiles. Results: The nonparametrically calculated reference values for TSH and fT4 were 0.58-3.49 mIU/L and 11.58-20.46 pmol/L, respectively. Statistically remarkable is the finding of a normal Gaussian distribution of the fT3 serum concentration, leading to the parametric reference interval of 3.56-5.88 pmol/L. Conclusions: The established reference values for this regional collective showed tighter intervals than the reference ranges provided by the manufacturer. A carefully selected study population, based on the correspondent National Academy of Clinical Biochemistry criteria, ensured a valid set of reference ranges for TSH, fT3, and fT4, providing a basis for accurate in vitro thyroid testing. The 2.5th percentile for the fT3 is now in better accordance with clinical findings.