Assessment of iodine nutritional status and gestational thyroid function reference ranges during the first trimester of pregnancy in Taiwan

Guan-Yu Su; Chang-Ching Yeh; Shun-Jie Yang; Chen-Chang Yang; Chii-Min Hwu; Fan-Fen Wang; Chun-Jui Huang

doi:10.1097/JCMA.0000000000001099

Assessment of iodine nutritional status and gestational thyroid function reference ranges during the first trimester of pregnancy in Taiwan

J Chin Med Assoc. 2024 Apr 23. doi: 10.1097/JCMA.0000000000001099. Online ahead of print.

Authors

Guan-Yu Su¹, Chang-Ching Yeh^{2

3

4}, Shun-Jie Yang¹, Chen-Chang Yang^{5

6

7

8}, Chii-Min Hwu^{1

5}, Fan-Fen Wang^{1

9}, Chun-Jui Huang^{1

5

6}

Affiliations

¹ Division of Endocrinology and Metabolism, Department of Medicine, Taipei Veterans General Hospital, Taipei, Taiwan, ROC.
² Department of Obstetrics & Gynecology, Taipei Veterans General Hospital, Taipei, Taiwan, ROC.
³ Department of Obstetrics and Gynecology, School of Medicine, College of Medicine, National Yang Ming Chiao Tung University, Taipei, Taiwan, ROC.
⁴ Department of Nurse-Midwifery and Women Health, College of Nursing, National Taipei University of Nursing and Health Sciences, Taipei, Taiwan, ROC.
⁵ School of Medicine, College of Medicine, National Yang Ming Chiao Tung University, Taipei, Taiwan, ROC.
⁶ Institute of Public Health, School of Medicine, National Yang Ming Chiao Tung University, Taipei, Taiwan, ROC.
⁷ Institute of Environmental & Occupational Health Sciences, School of Medicine, National Yang Ming Chiao Tung University, Taipei, Taiwan, ROC.
⁸ Department of Occupational Medicine and Clinical Toxicology, Taipei Veterans General Hospital, Taipei, Taiwan, ROC.
⁹ Department of Medicine, Taipei City Hospital Yangming Branch, Taipei, Taiwan, ROC.

PMID: 38651854
DOI: 10.1097/JCMA.0000000000001099

Abstract

Background: Iodine nutrition is critical for fetal neurodevelopment in the first trimester of pregnancy, a period associated with dramatic changes in thyroid function. The aim of this study was to evaluate iodine nutritional status and thyroid function reference ranges in the first trimester in Taiwan.

Methods: Pregnant women aged 20 years and above in the first trimester were recruited in Taipei Veterans General Hospital, Taiwan from March 2019 to July 2022. Each participant provided a spot urine sample for measurement of urinary iodine concentration (UIC) and a blood sample for check-up of thyroid function and thyroid autoantibodies. A simple food frequency questionnaire was also completed.

Results: A total of 209 women with a mean age of 32.9 ± 4.4 years were enrolled. The median UIC was 160.9 μg/L [interquartile range (IQR): 105.0-246.2 μg/L], indicating overall iodine sufficiency. The gestational thyroid function reference ranges were: TSH [median: 0.93 (0.007-2.9) µIU/mL], free T4 [1.3 (0.93-2.2) ng/dL], free T3 [3.0 (2.3-5.0) ng/dL], total T4 [9.9 (6.4-16.9) ng/dL] and total T3 [135 (88-231) ng/dL]. If the non-pregnant reference range of serum TSH was used, eight women (4.8%) would be misclassified as having subclinical hyperthyroidism, and two women (1.2%) with subclinical hypothyroidism would be missed. In multivariate analysis, nulliparous [adjusted odds ratio (OR) from model 1-3: 2.02, 2.05, 2.02; 95% confidence interval (CI): 1.08-3.77, 1.10-3.81, 1.11-3.66; p = 0.027, 0.023, 0.022, respectively] and multivitamin non-users (adjusted OR from model 1-3: 1.86, 1.85, 1.78; 95% CI: 1.04-3.34, 1.03-3.32, 1.004-3.71; p = 0.038, 0.039, 0.049, respectively) had increased odds of having lower UIC levels <150 μg/L.

Conclusion: The iodine nutritional status in the first trimester is adequate in Taiwan; however, certain subgroups such as nulliparous and multivitamin non-users are still at risk for iodine deficiency. Gestational thyroid function reference ranges are needed for correct diagnosis of thyroid dysfunction in pregnancy.