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Maternal Thyroid Function during the First Trimester of Pregnancy in Korean Women
Int J Thyroidol 2017;10(1):36-41
Published online May 30, 2017;
© 2017 Korean Thyroid Association.

Hyung Wook Choi1, You Jung Han2, Dong Wook Kwak2, So Young Park1, Sung Hoon Kim1, Hyun Koo Yoon1 and Chang Hoon Yim1

Department of Internal Medicine1, Obstetrics & Gynecology2, Cheil General Hospital & Women’s Healthcare Center, Dankook University College of Medicine, Seoul, Korea
Correspondence to: Chang Hoon Yim, MD, Department of Internal Medicine, Cheil General Hospital & Women’s Healthcare Center, Dankook University College of Medicine, 17 Seoae-ro 1-gil, Jung-gu, Seoul 04619, Korea
Tel: 82-2-2000-7248, Fax: 82-2-2264-1490, E-mail:
Received February 9, 2017; Revised April 25, 2017; Accepted April 27, 2017.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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.
Keywords : Pregnancy, Reference interval, Thyroid function