Dr. Sheldon Rubenfeld Board Certified in Internal Medicine and in Endocrinology, Diabetes, and Metabolism

Pregnancy and Thyroid Disease?

Confusion regarding thyroid function test results during pregnancy may arise because these results will vary depending on how far along women are in their pregnancies. Thyroid function test results, such as TSH and thyroid hormone (T4 and T3) levels, will fluctuate during normal pregnancies. These levels may go up and down and, yet, remain in the normal range for pregnant women. When pregnant women say that their hormones are bouncing all over the map, they are only scratching the surface of the problem. 

Several factors contribute to these fluctuations, including estrogens and human chorionic gonadotropin (hCG). Beginning in the early stages of pregnancy, estrogens cause an increase in thyroid hormone-binding proteins, which remain elevated until a few months after delivery. These increased thyroid hormone-binding proteins may cause a 10% to 15% decrease in free T4 and T3, even though they are usually within the normal range for pregnant women. Correspondingly, the TSH rises slightly between the first trimester and delivery.

During the first eight to fourteen weeks of pregnancy, hCG, a hormone produced by the placenta, rises significantly. If hCG rises sufficiently, it can mimic, to a mild degree, the function of TSH. Therefore, hCG can sometimes raise free T4 and T3 levels. Although the free T4 and T3 levels are still in the normal range, they may be sufficiently elevated to lower TSH levels during the first eight to fourteen weeks of pregnancy in up to 20% of pregnancies. These women have transient subclinical hyperthyroidism.  In a much smaller percentage of normal pregnancies, the effects of hCG can be so pronounced during this six-week period that free T4 may be temporarily elevated out of the normal range for pregnant women. These laboratory findings (an elevated free T4 and a low TSH) are sometimes referred to as gestational transient hyperthyroidism, estational thyrotoxicosis, or gestational hyperyroximenmia.

Hypothyroidism and Pregnancy -Women need approximately 45% more T4 during pregnancy to maintain a normal TSH level. However, the thyroid gland of a hypothyroid pregnant woman may not adequately increase production of these thyroid hormones. Therefore, even if she is taking levothyroxine, her TSH level may rise out of the normal range, indicating subclinical hypothyroidism.

In order to prevent even mild hypothyroidism during pregnancy, a hypothyroid woman should have her TSH checked shortly before she becomes pregnant and, then again, within six weeks of conception. Her TSH should then be rechecked approximately every six weeks during the remainder of her pregnancy. It is not unusual for the dosage of levothyroxine to increase early in the pregnancy and then change several times before delivery. Once a pregnant woman delivers, her TSH should be rechecked three to four months later.

Women with Hashimoto’s thyroiditis are predisposed to problems during pregnancy. For unknown reasons, women with Hashimoto’s thyroiditis are at higher risk of miscarriage than women without it. In addition, p