The correct reference range for maternal thyroid function during pregnancy is essential for making an accurate diagnosis of thyroid disease and delivering proper interventions in pregnant women. But there is still no universal standard for this in women with a twin pregnancy.

To determine a rational reference range for maternal thyroid function during twin pregnancies.

Healthy pregnant women who underwent antenatal examination in Obstetric Clinic, Beijing Friendship Hospital, Capital Medical University from January 2009 to September 2019 were retrospectively selected, including 352 with a twin pregnancy (twin group) , and 988 with a singleton pregnancy (singleton group) . Clinical and laboratory data were collected. The lower and upper limits for determining normal maternal thyroid function during twin pregnancies were the 2.5 (P_2.5) and 97.5 (P_97.5) percentiles of TSH and FT₄. Clinical hyperthyroidism was defined as TSH<P_{2.5 (total TSH)} and FT₄>P_{97.5 (total FT4)} . Clinical hypothyroidism was defined as TSH>P_{97.5 (total TSH)} and FT₄<P_{2.5 (total FT4)} . Subclinical hypothyroidism was diagnosed by TSH>P_97.5 and P_2.5≤FT₄≤P_97.5. Low T₄ syndrome was diagnosed by P_{2.5 (total TSH)} ≤TSH≤P_{97.5 (total TSH)} and FT₄<P_{2.5 (total FT4)} . FT₄ and TSH levels in the early, middle and late pregnancy were compared between singleton and twin groups. Prevalence of thyroid function abnormalities in the early, middle and late pregnancy was in twin group was recorded and analyzed.

Three hundred and fifty-two pregnant women with a twin pregnancy and 988 with a singleton pregnancy were finally included. The average FT₄ level in the twin group was higher than that of the singleton group regardless of the stage of pregnancy (P<0.05) . The average TSH level in the twin group was lower in the early pregnancy, but was higher in late pregnancy compared with that of singleton group (P<0.05) . For maternal thyroid function during a twin pregnancy, the determined normal FT₄ in the early, middle and late pregnancy expressed as median and interquartile range M (P_2.5, P_97.5) was 〔11.84 (7.95, 26.73) 〕, 〔8.24 (5.53, 18.58) 〕, 〔8.37 (5.80, 15.79) 〕pmol/L, respectively, and the determined normal TSH in the three stages of pregnancy was〔0.67 (0.03, 3.99) 〕, 〔1.44 (0.06, 4.79) 〕, 〔2.43 (0.41, 6.92) 〕mU/L, respectively. In the twin group, the prevalence of hyperthyroidism, clinical hypothyroidism, subclinical hypothyroidism, and low T₄ syndrome was 0, 0.28% (1/352) , 4.83% (17/352) and 3.98% (14/352) , respectively, by the above-mentioned criteria for diagnosing thyroid disease in a twin pregnancy, and that of the four diseases was 8.24% (29/352) , 0, 15.91% (56/352) and 1.99% (7/352) , respectively, by the criteria for diagnosing thyroid disease in a singleton pregnancy.

In this study, the recommended reference ranges of FT₄ in the early, middle and late stages of pregnancy were 7.95-26.73, 5.53-18.58 and 5.80-15.79 pmol/L, respectively, and the reference ranges of TSH were 0.03-3.99, 0.06-4.79 and 0.41-6.92 mU/L, respectively. Based on the FT₄ and TSH standards of the pregnant women with twin pregnancies obtained in our laboratory as the reference standards, the incidence of thyroid dysfunction detected in the pregnant women with twin pregnancies is low, which is consistent with relevant literature reports. The FT₄ and TSH standard range of single pregnancy obtained in our laboratory may lead to overdiagnosis of hyperthyroidism and subclinical hypothyroidism in pregnant women of twin pregnancy. So it is necessary to establish specific reference intervals for pregnant women with twin pregnancies based on the FT₄ and TSH standard ranges obtained in our laboratory.