Background:Serum human chorionic gonadotrophin (hCG) is higher in twin than that in singleton pregnancies.As hCG stimulates the thyroid to produce more free thyroxine (FT4),which may lead to decreased thyroid-stimulating hormone (TSH) levels,the reference ranges of thyroid-related indicators may differ between singleton and twin pregnancies in the first trimester.This study aimed to establish reference ranges for thyroid-related indicators in early twin pregnancies and to compare them with singleton pregnancies.Methods:Data of 820 twin-pregnant women were extracted from the established database of all pregnant women who delivered at Peking University First Hospital from October 2013 to May 2018;160 who met National Academy of Clinical Biochemistry criteria were included to establish TSH and FT4 reference ranges.We screened 480 (3:1 paired) women with singleton pregnancies from the same database as controls.The Mann-Whitney test for TSH and FT4 levels was applied for comparisons between singleton and twin pregnancies.Results:First-trimester reference ranges (4-12 gestational weeks) for twin pregnancies were:TSH 0.69 (0.01-3.35) mIU/L and FT4 16.38 (12.45-23.34) pmol/L.Median TSH was significantly lower at 7 to 12 gestational weeks than that at 4 to 6 gestational weeks (0.62 vs.0.96 mIU/L,Z =-1.964,P =0.049);FT4 was not significantly different between the two groups.Compared to singleton pregnancies,median TSH was significantly lower (0.69 vs.1.27 mIU/L,Z =-6.538,P =0.000),and FT4 was significantly higher (16.38 vs.14.85 pmol/L,Z =-7.399,P =0.000) in twin pregnancies in the first trimester.Conclusions:Specific reference ranges for thyroid-related indicators for twin pregnancies are needed to avoid a misdiagnosis of thyroid dysfunction.Moreover,establishment of separate reference ranges for 4 to 6 and 7 to 12 gestational weeks in twin pregnancies may be considered.