The pattern of thyroid function of subclinical hypothyroid women with levothyroxine treatment during pregnancy

In this study, we researched the pattern of thyroid function of subclinical hypothyroid (SCH) women with levothyroxine (LT₄) throughout pregnancy and determined the optimal dosages of LT₄ for such women. 56 SCH pregnant women were followed regularly prospectively. They were divided into three groups: group A (n = 29. Baseline TSH between 2.5 and 5.0 mIU/L) received 50 μg/day of LT₄; group B (n = 17. Baseline TSH between 5.0 and 8.0 mIU/L) received 75 μg/day of L-T₄; group C (n = 10. Baseline TSH >8.0 mIU/L) received 100 μg/day of LT₄. All the patients started LT₄ therapy around the 8th gestational week. Serum TSH, free T₄, and free T₃ were measured at 4-week intervals throughout pregnancy. Among the thyroid function indexes, serum TSH changed quickly and decreased significantly 4 weeks after LT₄ therapy. The change of serum FT₄ responded drastically at the 12th gestational week. We also found that serum FT₃ did not change much after LT₄ therapy. 50, 75, and 100 μg/day can maintain serum TSH level of 79.3, 82.4, and 90.0 % of SCH women in definitive therapeutic goal in group A, B, and C, respectively. The LT₄ dose need to be adjusted one or more times in 20.7, 17.6, and 10.0 % of the SCH women in group A, B, and C, respectively. The different LT₄ dose can be chosen according to the baseline TSH levels of SCH women. The constant LT₄ dose can maintain serum TSH levels of 79.3–90 % patients in the ideal range. A small part of patients require adjust therapy also during the second and third trimester. Therefore, careful follow-up of SCH pregnant women should be taken, and thyroid function should be tested every month until the end of pregnancy.