The aim of the present study was to investigate the serum levels of free triiodothyronine (FT3), free thyroxine (FT4) and thyroid stimulating hormone (TSH) during pregnancy and comparative analysis of serum markers levels in non-pregnant women and pregnant women. Pregnant women were divided into four groups according to their gestational age: 8-14, 15-20, 21-36 and ≥37 weeks. Non-pregnant women were divided into three groups according to their age: 20-40, 41-55 and 56-85 years, and women of reproductive age (20-40 years) as control, which match their age and body mass index with pregnant women. The levels of serum markers were measured by magnetic microparticle chemiluminescence immunoassay and compared among different gestational weeks or with the control. In pregnant women, from 8-14 to ≥37 weeks, FT3 and FT4 levels declined, and significant differences were identified between each group, except for FT4 at 21-36 weeks, when compared with ≥37 weeks. While TSH was increased and significant differences were identified between each group. Compared with the control group, the concentrations of FT3, FT4 and TSH were lower in pregnant women for all weeks except for TSH in ≥37 weeks. Reference intervals of FT3, FT4 and TSH in pregnant women are much lower than that of the control women. Pregnant women are likely to have lower thyroid hormone levels throughout pregnancy. The current reference intervals of thyroid hormones were not feasible for pregnant women and pregnancy-specific reference intervals should be established according to a local Chinese pregnant women database.
Keywords: free thyroxine and thyroid stimulating hormone; free triiodothyronine; pregnancy; thyroid function.