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

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.