Objective  To investigate gestational thyroid function in women with twin pregnancy and compare with that among the women with singleton pregnancy.

  Methods  Data on serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), thyroid peroxidase antibody (TPOAb) for 19 979 pregnant women, including 317 women with twin pregnancy, were extracted from pregnant health records in Tianjincity form January 2104 through December 2015. We analyzed the differences in serum TSH and FT4 levels between the women with single and twin pregnancy.

  Results  The serum TSH level (median = 1.05 mIU/L) of the women with twin pregnancy was significantly lower than that (median = 1.35 mIU/L) of women with singleton pregnancy (P < 0.01). The abnormal rate (23.66%) of thyroid function among the women with twin pregnancy was higher than that (16.61%) among the women with singleton pregnancy, with a significantly increased rate of hyperthyroidism (P < 0.01). The changing trend of serum TSH level with gestational weeks in the women with twin pregnancy was similar to that in the women with singleton pregnancy; but the serum TSH level of the women with twin pregnancy was significantly lower than that of the women with singleton pregnancy during the period of 8 – 20th gestational week (P < 0.05) and the level was even higher after 28th week. The serum FT4 level in the women with twin pregnancy was higher than that in the women with singleton pregnancy at 8th and till 12th gestational week (P < 0.05). Logistic regression results revealed that twin pregnancy was associated with increased risk of hyperthyroidism compared to the singleton pregnancy (odds ratio = 4.082, 95% confidence interval: 2.936 – 5.675).

  Conclusion  The serum TSH level is lower while thehyperthyroidism prevalence is higher among the women with twin pregnancy than in the women with singleton pregnancy.