Periconceptional changes in thyroid function: a longitudinal study

Women (N = 60) ages 30-42 years without a history of thyroid disease, who were planning pregnancy, were observed prospectively before and during early pregnancy. Thyroid function (thyroid stimulating hormone, TSH and free thyroxine, FT4) was measured before conception and between 6 and 9 weeks gestation. Pre-pregnancy samples were analyzed for thyroid antibodies. Bivariate analyses and longitudinal curves (general estimating equation models) were used to analyze changes in thyroid function during the periconception window by antibody status.Pre-pregnancy TSH values were significantly higher than early pregnancy TSH (p < 0.001), but FT4 values did not differ (p = 0.53). TSH declined as gestational age increased (P < 0.01). Thyroid antibody positive women had a higher pre-pregnancy TSH compared to antibody negative women (p < 0.01). Periconceptional change in thyroid function was more variable among women with antibodies (p < 0.001). 50% of women with elevated pre-pregnancy TSH values (TSH > 3.0 mIU/L) had normal TSH values (TSH < 2.5 mIU/L) in pregnancy.TSH values decline during the transition from pre-pregnancy to early pregnancy. The change in TSH appears to be less predictable in women with thyroid antibodies. Periconceptional changes in thyroid function should be considered in formulating prenatal thyroid screening guidelines.Reproductive hormones have been shown to impact thyroid physiology during pregnancy [1-3]. Estrogens stimulate thyroid binding globulin (TBG) production by the liver effectively decreasing free thyroxine (FT4) [1]. Human chorionic gonadotropin binds to and activates the thyroid stimulating hormone (TSH) receptor [2,3]. Therefore, high levels of reproductive hormones produced during pregnancy likely lead to alterations in maternal thyroid function and measures of thyroid function (TSH and FT4 levels). Normal maternal thyroid function during the periconception window defined as the transition from pre-pregnancy through the early first trimeste