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 (LT 4 ) throughout pregnancy and determined the optimal dosages of LT 4 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 LT 4 ; group B ( n = 17. Baseline TSH between 5.0 and 8.0 mIU/L) received 75 μg/day of L-T 4 ; group C ( n = 10. Baseline TSH >8.0 mIU/L) received 100 μg/day of LT 4 . All the patients started LT 4 therapy around the 8th gestational week. Serum TSH, free T 4 , and free T 3 were measured at 4-week intervals throughout pregnancy. Among the thyroid function indexes, serum TSH changed quickly and decreased significantly 4 weeks after LT 4 therapy. The change of serum FT 4 responded drastically at the 12th gestational week. We also found that serum FT 3 did not change much after LT 4 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 LT 4 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 LT 4 dose can be chosen according to the baseline TSH levels of SCH women. The constant LT 4 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.