Journal List > Int J Thyroidol > v.10(1) > 1082702

Int J Thyroidol. 2017 May;10(1):36-41. Korean.
Published online May 26, 2017.
Copyright © 2017. the Korean Thyroid Association
Maternal Thyroid Function during the First Trimester of Pregnancy in Korean Women
Hyung Wook Choi,1 You Jung Han,2 Dong Wook Kwak,2 So Young Park,1 Sung Hoon Kim,1 Hyun Koo Yoon,1 and Chang Hoon Yim1
1Department of Internal Medicine, Cheil General Hospital & Women's Healthcare Center, Dankook University College of Medicine, Seoul, Korea.
2Department of Obstetrics & Gynecology, Cheil General Hospital & Women's Healthcare Center, Dankook University College of Medicine, Seoul, Korea.

Correspondence: 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, Email:
Received February 09, 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).


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.


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


Fig. 1
Serum thyroid stimulating hormone variation according to gestational week in 8365 Korean pregnant women.
Click for larger image


Table 1
Percentile values of TSH of non-pregnant and each gestational age pregnant women
Click for larger image

Table 2
Prevalence of hypothyroidism with different diagnostic criteria
Click for larger image

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