Hypothyroid women who experience increased estrogen levels, either due to pregnancy or estrogen replacement therapy, often need higher doses of thyroid replacement hormone. While this is primarily due to estrogen-induced increases in the proteins that bind thyroid hormone, other factors may also be involved. Because of these changes, during the second trimester of pregnancy, most hypothyroid women need an increased dosage of thyroid replacement hormone. And women who begin hormone replacement therapy often need an increased dose of thyroid replacement hormone. Otherwise, they may experience symptoms of hypothyroidism.
In pregnancy, the fetus is dependent on the mother for adequate thyroid hormone. If the dose of replacement hormone isn’t adjusted appropriately, fetal health may be impaired. Even slight hypothyroidism in the mother can result in mental impairment in the fetus.
According to a recent article in the New England Journal of Medicine, women with hypothyroidism need an average of 45 percent more thyroxine (T4) during pregnancy to maintain euthyroidism. In normal or euthyroid women receiving estrogen replacement hormone, thyroid function tests remain unchanged. However in hypothyroid women on thyroid replacement hormone, the addition of estrogen therapy caused decreased blood levels of T4. Although the changes were small, they are considered clinically important. Researchers noted that the increased TSH level caused by estrogen therapy resulted in symptoms of hypothyroidism in a number of women in the study.
The study’s authors noted that these findings correlate with the improved thyroid function often reported in Graves’ disease patients who begin estrogen therapy, including birth control pills. Estrogen raises the serum thyroxine-binding globulin (protein that carries thyroid hormone in the blood) concentration by slowing its clearance from the blood and by enhancing its production in the body.
However, estrogen can stimulate the immune system, causing an increased production of thyroid antibodies. Since the effects of estrogen are variable, women should also be on the alert for signs of hyperthyroidism.
Progesterone has an effect opposite of estrogen. It can decrease levels of thyroxine binding globulin, increasing thyroid hormone levels in the blood. However, it has a benign or protective effect on the immune system. Depending on the dose of progesterone taken by women using combined hormone replacement therapy, the effects of progesterone may balance out those of estrogen.
Because the effects of these hormones are dependent on genetic as well as environmental factors, it’s important that any women experiencing changes in their concentrations of estrogen and/or progesterone be monitored vigilantly for changes in thyroid function.
