Maternal thyroid function in multifetal pregnancies before and after fetalreduction

The aim of the study was to investigate maternal thyroid function in pregna ncy by monitoring the circulating concentrations of thyroid stimulating hor mone (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) pregnan cies achieved after superovulation and in vitro fertilisation and embryo tr ansfer (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 twirl pregnancies (singleton, mean 16.49 pmol/l (interquartil e 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 mul tiple regression analysis, fT(4) was significantly related to thr number of fetuses (F=23.739, P=0.0001), but not to hCG. After fetal reduction to twi ns, the circulating concentrations of fT(4) in multifetal pregnancies decre ased progressively towards those in control twin pregnancies, but remained significantly greater at both 4 (P=0.003) and 8 weeks (P=0.050). This patte rn 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. thi s may represent a delayed thyroid response to the decreasing concentrations of hCG, but the alternative is that the maternal thyroid function is contr olled by a fetal factor in addition to hCG.