The aim of the study was to investigate maternal thyroid function in pregnancy by monitoring the circulating concentrations of thyroid stimulating hormone (TSH), free thyroxine (fT(4)) and human chorionic gonadotrophin (hCG) in multifetal pregnancies before and after embryo reduction. We studied two groups of women: group 1 comprised singleton (n=12) and twin (n=12) pregnancies achieved after superovulation and in vitro fertilisation and embryo transfer (IVF-ET), and group 2 were multifetal pregnancies (n=39) undergoing selective fetal reduction to twin pregnancies. Blood samples were obtained initially at 10-12 weeks gestation (before fetal reduction) and then 4 and 8 weeks afterwards. Before fetal reduction, the circulating concentrations of fT(4) in multifetal pregnancies were significantly greater than those in singleton or twin pregnancies (singleton, mean 16.49 pmol/l (interquartile range 14.09-18.13 pmol/l); twins, 15.84 (15.36-16.95 pmol/l); multifetal, 21.08 (16. 64-26.29 pmol/l); P<0.005 for singleton and twins), and in a multiple regression analysis, fT(4) was significantly related to the number of fetuses (F=23.739, P=0.0001), but not to hCG. After fetal reduction to twins, the circulating concentrations of fT(4) in multifetal pregnancies decreased progressively towards those in control twin pregnancies, but remained significantly greater at both 4 (P=0.003) and 8 weeks (P=0.050). This pattern of change in the concentrations of fT(4) is similar to, but lags behind, that of hCG, which attains twin levels 4 weeks after fetal reduction. This may represent a delayed thyroid response to the decreasing concentrations of hCG, but the alternative is that the maternal thyroid function is controlled by a fetal factor in addition to hCG.

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