Thyroid function and autoimmunity in pregnancy: contributing factors and a real-life performance of reference ranges recommended by the national guidelines: results of the longitudinal study

INTRODUCTION: Thyroid dysfunction and thyroid autoimmunity during pregnancy are linked to an increased risks of adverse maternal and fetal outcomes. Guidelines recommend population‑specific thyroid hormone reference ranges in pregnancy, though adherence and effectiveness vary. OBJECTIVES: We aimed to evaluate the performance of thyroid function reference ranges recommended by Polish national guidelines in pregnancy and to assess clinical and demographic factors affecting thyroid hormone levels and autoimmunity. PATIENTS AND METHODS: A cohort of 1339 pregnant women across all trimesters was recruited in Poland (2007-2017). Serum levels of thyroid‑stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), and thyroid peroxidase antibodies (TPOAbs), and urinary iodine concentration (UIC) were measured. Associations between thyroid status and maternal age, body mass index (BMI), gestational age, and UIC were analyzed. RESULTS: Over 91% of the participants had TSH concentrations within Polish trimester‑specific reference ranges. TPOAbs were positive in 12% of the cases, and were strongly associated with higher TSH and lower FT4 concentrations (P