Monitoring Thyroxine Treatment During Pregnancy

Abstract

Data were analyzed from 77 pregnancies in 65 hypothyroid women treated with Ievothyroxine (T₄) to determine (a) how often, how severely, and when serum thyrotropin (TSH) and free T4 concentrations become abnormal during pregnancy in women taking constant T₄ doses and (b) how much to increase T₄ doses to normalize elevated serum TSH levels. Group I consisted of 36 women with previous thyroid ablation. Group II consisted of 29 women with Hashimoto's thyroiditis. Serum TSH levels rose above normal (> 4 μU/mL) during pregnancy more often in Group I (76%) than in Group II (47%) during treatment with constant T₄ doses, and mean serum free T₄ levels decreased significantly in both groups, by 47% in Group I and by 35% in Group II, compared to preconception values. Serum TSH concentrations rose above 20 μU/mL in 22% of the women, and serum free T₄ concentrations fell below normal in 13%. Elevated serum TSH levels usually were detected by the first TSH test during pregnancy, as early as 4 weeks gestation. Of those women tested after an initial normal serum TSH value during pregnancy, 30% had an elevated serum TSH concentration later in the pregnancy. Of 12 women followed through two pregnancies, 10 had elevated serum TSH values in both pregnancies, 1 had normal serum TSH values in both, and 1 had discordant serum TSH values. Mean (± SD) T₄ dose increments needed to normalize serum TSH values were 41 ± 24 μg T₄/day for elevated serum TSH values <10 μU/mL, 65 ± 19 μg/day for serum TSH values between 10 and 20 μU/mL, and 105 ± 32 μg/day for serum TSH values >20 μU/mL. Optimal postpartum T₄ doses were approximately the same as those that produced normal serum TSH levels before conception. From these findings, it seems reasonable to advise pregnant hypothyroid women treated with T₄ to have serum TSH tests at 8 weeks and 6 months gestation. An elevated TSH value should be corrected by an increased T₄ doses. The required dose increment can be estimated from the abnormal TSH value. After pregnancy, optimal preconception doses can be resumed.