Thyroid hormones are crucial for metabolism, growth, and development in the human body, especially playing a pivotal role in fetal development during early pregnancy. Currently, research on thyroid hormone reference ranges for twin pregnancies remains inconsistent, and most studies on thyroid dysfunction and pregnancy outcomes in twin pregnancies still use singleton pregnancy reference ranges as a benchmark.

To establish reference intervals for thyroid stimulating hormone (TSH) and free thyroxine (FT₄) in early pregnancy among women with twin pregnancies, compare the incidence of thyroid dysfunction diagnosed based on singleton versus twin standards, and compare pregnancy outcomes for different thyroid function statuses under the two standards.

Twin pregnant women who visited Beijing Obstetrics and Gynecology Hospital, Capital Medical University, from October 2015 to February 2021 and met the inclusion and exclusion criteria were enrolled. We used chemiluminescence to detect TSH and FT₄ in early pregnancy, and determined the reference range based on bilateral limits of P_2.5-P_97.5. The reference interval for thyroid function in the first trimester of single pregnancy in our hospital is TSH 0.59-3.54 mU/L and FT₄ 11.8-18.4 pmol/L, respectively. Thyroid function results in early pregnancy for twin pregnancies were classified using both singleton and twin reference standards, including normal thyroid function, subclinical hypothyroidism, hypothyroidism, subclinical hyperthyroidism, and hyperthyroidism. Given the limited sample size, patients with hypothyroidism and subclinical hypothyroidism were combined into the hypothyroidism group, and those with hyperthyroidism and subclinical hyperthyroidism were combined into the hyperthyroidism group for statistical analysis. Differences in pregnancy outcomes for thyroid dysfunction diagnosed using different reference standards were analyzed. Binary Logistic regression was used to analyze the correlations between first-trimester thyroid function reference standards and gestational diabetes mellitus, hypertensive disorders of pregnancy, as well as preterm birth. Multiple linear regression was applied to assess the associations of first-trimester thyroid function reference standards with gestational weight gain, neonatal birth weight, and neonatal TSH.

A total of 1 724 twin pregnant women were enrolled, of which 254 met the inclusion and exclusion criteria for establishing reference intervals for thyroid function in early twin pregnancies. The TSH range for twin pregnancies in early pregnancy was 0.11-3.16 mU/L, and the FT₄ range was 12.45-28.56 pmol/L. Using the single reference standard for classification, 2 cases (0.1%) of clinical hypothyroidism were diagnosed, 33 cases (1.9%) of subclinical hypothyroidism, 860 cases (49.9%) of normal, 363 cases (21.1%) of subclinical hyperthyroidism, 366 cases (21.2%) of clinical hyperthyroidism, and 100 cases (5.8%) of other. Using the twin reference standard for classification, 3 cases (0.2%) of clinical hypothyroidism were diagnosed, 47 cases (2.7%) of subclinical hypothyroidism, 1 609 cases (93.3%) of normal, 3 cases (0.2%) of subclinical hyperthyroidism, 3 cases (0.2%) of clinical hyperthyroidism, and 59 cases (3.4%) of other. Using different reference standards for thyroid function in the first trimester of pregnancy for singleton and twin pregnancies, there was a statistically significant difference in the classification of thyroid function in the first trimester of twin pregnancy (P<0.001). Among pregnant women with normal thyroid function, the pre-pregnancy BMI and early pregnancy TSH in the singleton reference standard were higher than those in the twin reference standard, while the early pregnancy FT₄ was lower than that in the twin reference standard (P<0.05). Among pregnant women with hypothyroidism, the early pregnancy TSH in the singleton reference standard was higher than that in the twin reference standard (P<0.05). Among pregnant women with hyperthyroidism, the early pregnancy TSH, total weight gain during pregnancy, and neonatal birth weight in the singleton reference standard were higher than those in the twin reference standard, while the early pregnancy FT₄ was lower than that in the twin reference standard (P<0.05). The results of regression analysis showed that among pregnant women with hyperthyroidism, different reference standards for thyroid function were associated with gestational weight gain and neonatal birth weight (P<0.05).

The TSH reference range for early twin pregnancies at our center is 0.11-3.16 mU/L, and the FT₄ reference range is 12.45-28.56 pmol/L. The number of cases of subclinical hyperthyroidism and hyperthyroidism diagnosed using the twin reference standard is significantly lower than that using the single reference standard. In pregnant women with hyperthyroidism, the reference standards for thyroid function are related to weight gain during pregnancy and the birth weight of the newborn.