Hyperthyroidism in pregnancy is usually caused by Graves’ disease and occurs in about one of every 500 pregnancies.
What causes hyperthyroidism in pregnancy?
Hyperthyroidism in pregnancy is usually caused by Graves’ disease and occurs in about one of every 500 pregnancies. Graves’ disease is an autoimmune disorder, which means the body’s immune system makes antibodies that act against its own healthy cells and tissues. In Graves’ disease, the immune system makes an antibody called thyroid stimulating immunoglobulin, sometimes called TSH receptor antibody, which mimics TSH and causes the thyroid to make too much thyroid hormone.
Although Graves’ disease may first appear during pregnancy, a woman with preexisting Graves’ disease could actually see an improvement in her symptoms in her second and third trimesters. Remission of Graves’ disease in later pregnancy may result from the general suppression of the immune system that occurs during pregnancy. The disease usually worsens again in the first few months after delivery.
Rarely, hyperthyroidism in pregnancy is caused by hyperemesis gravidarum—severe nausea and vomiting that can lead to weight loss and dehydration. This extreme nausea and vomiting is believed to be triggered by high levels of hCG, which can also lead to temporary hyperthyroidism that usually resolves by the second half of pregnancy.
How does hyperthyroidism affect the mother and baby?
Uncontrolled hyperthyroidism during pregnancy can lead to
- congestive heart failure
- preeclampsia—a dangerous rise in blood pressure in late pregnancy
- thyroid storm—a sudden, severe worsening of symptoms
- premature birth
- low birthweight
If a woman has Graves’ disease or was treated for Graves’ disease in the past, the thyroid-stimulating antibodies she produces may travel across the placenta to the baby’s bloodstream and stimulate the fetal thyroid. If the mother is being treated with antithyroid drugs, hyperthyroidism in the baby is less likely because these drugs also cross the placenta. But if she was treated for Graves’ disease with surgery or radioactive iodine, both of which destroy all or part of the thyroid, she can still have antibodies in her blood even though her thyroid levels are normal. Women who received either of these treatments for Graves’ disease should inform their doctor so the baby can be monitored for thyroid-related problems later in the pregnancy.
Hyperthyroidism in a newborn can result in rapid heart rate that can lead to heart failure, poor weight gain, irritability, and sometimes an enlarged thyroid that can press against the windpipe and interfere with breathing. Women with Graves’ disease and their newborns should be closely monitored by their health care team.
How is hyperthyroidism in pregnancy diagnosed?
Hyperthyroidism is diagnosed through a careful review of symptoms, as well as blood tests to measure TSH, T4, and T3 levels.
Some symptoms of hyperthyroidism are common features in normal pregnancies, including increased heart rate, heat intolerance, and fatigue. Other symptoms are more indicative of hyperthyroidism: rapid and irregular heartbeat, a fine tremor, unexplained weight loss or failure to have normal pregnancy weight gain, and the severe nausea and vomiting associated with hyperemesis gravidarum.
If a woman’s symptoms suggest hyperthyroidism, her doctor will probably first perform the ultrasensitive TSH test. This test detects even tiny amounts of TSH in the blood and is the most accurate measure of thyroid activity available. Generally, below-normal levels of TSH indicate hyperthyroidism. Low TSH levels may also occur in a normal pregnancy, however, especially in the first trimester. If TSH levels are low, another blood test is performed to measure T4 and T3.
Elevated levels of free T4—the portion of thyroid hormone not attached to thyroid-binding proteins—confirm the diagnosis. Rarely, free T4 levels are normal in a woman with hyperthyroidism but T3 levels are high. Because of normal pregnancy-related changes in thyroid function, test results must be interpreted with caution.
If a woman has Graves’ disease or has had surgery or radioactive iodine treatment for the disease, her doctor may also test her blood for the presence of thyroid-stimulating antibodies.
How is hyperthyroidism treated during pregnancy
Mild hyperthyroidism in which TSH is low but free T4 is normal does not require treatment. More severe hyperthyroidism is treated with propylthiouracil or sometimes methimazole (Tapazole), drugs that interfere with thyroid hormone production. Antithyroid drugs cross the placenta in small amounts and can decrease fetal thyroid hormone production, so the lowest possible dose should be used to avoid hypothyroidism in the baby.
Rarely, surgery to remove all or part of the thyroid gland is considered for women who cannot tolerate propylthiouracil or methimazole. Radioactive iodine treatment is not an option for pregnant women because it can damage the fetal thyroid gland.
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