Development of Gestational Age-Specific Thyroid Function Test Reference Intervals in Four Analytic Platforms Through...

A population-based reference interval (RI) of thyroid hormones in pregnancy using a standardized methodology is crucial for clinicians to make accurate diagnoses and important for the comparison of test results obtained from different analytic platforms. We enrolled 600 healthy Chinese women to obtain longitudinal serum samples across gestation, after exclusion of subjects with antibodies to thyroid peroxidase, thyroglobulin or thyrotropin receptor. Gestational age-specific RIs were constructed by using polynomial regression equations with MLwiN. Free thyroxine (fT4) levels rose to a peak at the 7th-8th gestational weeks and then declined gradually till 28th week, while thyrotropin (TSH) level decreased from early pregnancy to a nadir at the 9th week. The data support the recent notion by the American Thyroid Association to raise the TSH upper RI to 4.0 mIU/L. We also demonstrate that thyroid hormone reference ranges are not affected in a mildly iodine-deficient population and by including women with the presence of antibodies against thyroid peroxidase and thyroglobulin who are otherwise healthy. The study highlights a methodology in constructing gestational age-specific thyroid function test RIs on different analytic platforms to provide a better interpretation and comparison of results obtained across different platforms.