Background 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. Objective To determine a rational reference range for maternal thyroid function during twin pregnancies. Methods Healthy pregnant women who underwent an 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（P2.5） and 97.5 （P97.5） percentiles of TSH and FT4. Clinical hyperthyroidism was defined as TSHP97.5（total FT4）. Clinical hypothyroidism was defined as TSH>P97.5（total TSH） and FT4P97.5 and P2.5 ≤ FT4 ≤ P97.5. Low T4 syndrome was diagnosed by P2.5（total TSH） ≤ TSH ≤ P97.5 （total TSH） and FT4
"Reference Range for Thyroid Function during Twin Pregnancies,"
General Practice in China: Vol. 1:
1, Article 31.
Available at: https://www.gpinchina.net/journal/vol1/iss1/31