Researchers from Brigham and Women’s Hospital (BWH) found that women currently taking thyroid hormones need to increase their dose early in a pregnancy – on average, by eight weeks gestation – to prevent maternal hypothyroidism and possible harm to the fetus. These findings, which will be published in the July 15 issue of The New England Journal of Medicine, provide physicians – including endocrinologists, obstetricians and gynecologists, and primary care physicians – with new evidence that the necessary dosage of thyroid medication increases shortly after conception, and often prior to a woman’s first obstetrical visit.
Hypothyroidism, a deficiency of the thyroid hormone, is found in more than 5 million Americans, with as many as 10 percent of all women afflicted with some degree of thyroid hormone deficiency. Thyroid medications such as levothyroxine are essentially identical to the thyroid hormone made by the normal thyroid gland and therefore perfectly safe to take during pregnancy.
An estimated 1 to 2 percent of all pregnant women receive levothyroxine therapy for hypothyroidism. Given that maternal hypothyroidism is associated with an increased risk of premature delivery, placental abruption, and pregnancy-induced hypertension, as well as a probable increased risk of cognitive and developmental deficiencies in the developing fetus, women with a history of thyroid dysfunction must be counseled to adjust their thyroid hormone dose as soon as pregnancy is confirmed.
According to the study’s lead author, Erik K. Alexander, associate physician at BWH and instructor of medicine at Harvard Medical School, “At a minimum, an estimated 16,000 to 32,000 pregnant women annually may be impacted by this data.
Pregnancy causes an estimated 30 to 50 percent increase in the daily requirement of thyroid hormone. What has been unclear is at what point the increased requirement occurs during gestation. It appears that thyroid hormone doses need to be adjusted as soon as pregnancy is confirmed, usually prior to the first obstetrical visit.”
In this study, Alexander and his colleagues monitored 19 women with known hypothyroidism throughout pregnancy, with the goal of identifying the precise timing and pattern of increased thyroid hormone requirement during gestation.
Among this population, 20 pregnancies were recorded that resulted in 17 full-term births. The researchers found that 85 percent of pregnancies required a nearly 50 percent increase in the dosage of levothyroxine, and this occurred almost exclusively during the first trimester of the pregnancy. Typically, the first dose change occurred at eight weeks, but occurred even earlier in several patients – especially those pregnant via assisted reproductive techniques.
Alexander also found that levothyroxine requirements plateaued at 16 weeks, pointing to the need for stringent monitoring of blood thyroid hormone levels during early pregnancy, with less monitoring midway through pregnancy until delivery.