Increased need for thyroxine during pregnancy in women with primary hypothyroidism

Abstract

Abstract Background and Methods. Women with hypothyroidism have been thought not to require an increase in thyroxine replacement during pregnancy. To evaluate the effects of pregnancy on thyroxine requirements, we retrospectively reviewed the thyroid function of 12 women receiving treatment for primary hypothyroidism before, during, and after pregnancy. Results. In all patients, the serum thyrotropin level increased during pregnancy. The mean (±SE) serum freethyroxine index decreased from 111.0±5.8 before pregnancy to 86.5±5.2 during pregnancy (normal, 64 to 142; P<0.05), and the mean serum thyrotropin level increased from 2.0±0.5 mU per liter before pregnancy to 13.5±3.3 mU per liter during pregnancy (normal, 0.5 to 5.0 mU per liter; P<0.01). Because of high thyrotropin levels, the thyroxine dose was increased in 9 of the 12 patients. Among the three patients who did not require an increased thyroxine dose were two with low serum thyrotropin levels before pregnancy, suggesting excessive replacement at th...