Monitoring thyroxine treatment during pregnancy.

Data were analyzed from 77 pregnancies in 65 hypothyroid women treated with levothyroxine (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 (greater than 4 microU/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 microU/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.(ABSTRACT TRUNCATED AT 250 WORDS)