Reference Range of Thyroid Function Tests (Total Triiodothyronine, Total Thyroxine, and Thyroid-Stimulating Hormone) Among Iranian Adults

Background and aims: Thyroid hormones are the primary tests for diagnosing thyroid disorders in individuals. This study aimed to evaluate the reference ranges of serum thyroid-stimulating hormone (TSH), total thyroxine (T4), and total triiodothyronine (T3) among adults in Yazd province. Methods: This cross-sectional study was conducted on individuals participating in the Yazd Health Study. Overall, 1,943 participants were included according to the National Academy of Clinical Biochemistry criteria. The direct method was employed to establish accurate and relevant reference ranges. Moreover, the 2.5th and 97.5th percentiles were calculated, with 95% confidence intervals for the lower and upper limits. The obtained data were analyzed using MedCalc 12.1 and SPSS 25. Results: The mean±standard deviation age of the participants was 42.8±11.6 years. In addition, the reference intervals derived for TSH, T4, and T3 were 0.32–4.22 mU/L, 9.54–11.93 pmol/L, and 92.48–194.41 nmol/L, respectively. Furthermore, the 2.5th and 97.5th percentiles for TSH were 1.32 mU/L and 4.60 mU/L, respectively. Significant differences were observed in TSH between genders (P=0.001), while differences in T3 (P=0.146) and T4 (P=0.087) values between genders were not statistically significant. Conclusion: This study established age-specific and gender-specific reference ranges for T3, T4, and TSH in a large cohort of Iranian adults. Such information can help healthcare professionals categorize patients more accurately by thyroid function levels.