Pattern of thyroid function during early pregnancy in women diagnosed with subclinical hypothyroidism and treated with l-thyroxine is similar to that in euthyroid controls

BACKGROUND: Subclinical hypothyroidism (SCH) is associated with a higher miscarriage rate. It is unclear how the thyroid function in SCH differs from that in euthyroidism during early pregnancy. We intended to determine the regulation of thyroid function in women with SCH receiving constant l-thyroxine (T4) replacement during early pregnancy as compared to euthyroid controls. METHODS: This was a prospective cohort study with weekly serum sampling in eight women in early pregnancy with SCH and eight euthyroid women from week 5 to week 12 of pregnancy. Thyroid function was assessed before pregnancy. Women with SCH were treated with T4 (50 microg daily) and continued on an unchanged dose until week 12. The following parameters were measured weekly: thyrotropin (TSH), thyroglobulin, thyroxine, triiodothyronine, free thyroxine (FT4), free triiodothyronine (FT3), estradiol, progesterone, human chorionic gonadotropin, and prolactin. RESULTS: Although the pregestational levels of TSH were significantly higher among women with SCH as compared to euthyroid controls, the self-limited estrogen-induced increment of TSH during early pregnancy was similar in both groups. CONCLUSIONS: Although both SCH and ovarian hyperstimulation were associated with an intermediate rise in TSH, the pattern of thyroid function followed similar changes as in euthyroid controls and is unlikely to cause the higher miscarriage rate observed in SCH.