RESUMEN
BACKGROUND AND OBJECTIVES: Thyroid dysfunction during pregnancy can result in many complications for both mother and infant. Due to the physiologic changes in thyroid stimulating hormone (TSH) level during early pregnancy, it is recommend to use trimester-specific reference ranges for every population. We obtained the reference range of TSH during the first trimester in Korean women according to gestational week. MATERIALS AND METHODS: The study population consisted of pregnant women who had undergone a TSH screening during the first trimester of pregnancy (n=8365) and nonpregnant women (n=1835). RESULTS: Median concentration of serum TSH decreased significantly from the 5th to 8th week of gestation (median TSH concentration: 2.00 mIU/L for 5 weeks; 1.70 mIU/L for 6 weeks; 1.40 mIU/L for 7 weeks; 1.05 mIU/L for 8 weeks). However, there was no significant difference in median concentration of serum TSH from the 8th to 12th weeks of gestation. Using the fixed cut-off value of TSH >3.66 mIU/L, the diagnosis rate of subclinical hypothyroidism was 15.0% for 5 weeks, 10.0% for 6 weeks, 5.9% for 7 weeks, and 3.6% for 8-12 weeks. CONCLUSION: When interpreting the TSH test for pregnancy (maternal thyroid function), we should consider that the TSH level decreases significantly during the early first trimester.