Hyperthyroidism and Pregnancy
By Donald Aulds, M.D.
Hyperthyroidism (high thyroid hormones) in pregnancy occurs in approximately one out of every 1,500 pregnancies. The most common cause of hyperthyroidism is Graveís disease. It is important to consider other causes of hyperthyroidism such as elevated HCG (the pregnancy hormone in early pregnancy) levels due to severe nausea and vomiting experienced by some women during pregnancy, which can cause a temporary elevation of the thyroid hormone. In women with hyperthyroidism, menstrual periods may be irregular and ovulation may not occur. Once a woman is pregnant, there are increased risks of miscarriages, decreased fetal growth, premature delivery, pre-eclampsia and birth defects of the fetus. The risks of these problems decrease when hyperthyroidism is treated appropriately.
How is hyperthyroidism diagnosed?
Symptoms of hyperthyroidism are often difficult to distinguish from symptoms of pregnancy. Some of the symptoms include hot feelings, excessive sweating, emotional changes, nervousness and an increase in heart rate. If the heart rate is above 100 and stays elevated for a long period of time and a pregnant women experiences weight loss, hyperthyroidism must be considered. Radioactive iodine scans are contradicted in pregnancy so lab testing of the thyroid hormones must be done.
What are the risks of prolonged hyperthyroidism to the fetus?
Risks have to be considered based on the following conditions:
- Uncontrolled hyperthyroidism of the mother may result in rapid heart rate in the fetus, decreased fetal growth, premature labor and delivery, fetal deaths and birth defects.
- High levels of thyroid-stimulating immunoglobulins (TSI). Most hyperthyroidism is causes by an autoimmune disease where antibodies made by the body stimulate the over production of the thyroid hormones. These antibodies may cross the placenta and stimulate the babyís thyroid production, causing fetal hyperthyroidism. The TSI may stay in the blood of the baby for up to ten days after delivery, so the baby has to be checked for hyperthyroidism. If the baby has the disease and is not recognized and treated, fetal death may occur in up to 20 percent of babies with the disease.
- Anti-thyroid drugs. In the U.S. the drugs used to treat hyperthyroidism are Tapazole (methinazole) and PTU (propylthiouracil). The drugs can cross the placenta and suppress the thyroid production of the fetus, producing fetal hypothyroidism and fetal goiter. However, the overall benefits of the medications outweigh the risks.
What are the risks to the mother?
Undiagnosed and untreated hyperthyroidism in pregnancy is dangerous to the mother also. The most serious complications include heart failure as a result of the heart beating faster than normal, which overworks the heart muscle, and a condition called thyroid storm in which the thyroid hormone spirals out of control. Thyroid storm can be triggered by stresses of labor, surgery or infection and may lead to death.
What therapies are available for hyperthyroidism in pregnancy?
Many of the therapies available to control hyperthyroidism can be harmful to the developing fetus or may pass through the milk while breastfeeding and reduce thyroid production in the baby. The main goals of therapy are to protect both the mother and developing fetus. Two medications that are commonly used to treat non-pregnant women are beta-blockers to reduce heart rate and iodides. However, these are not used in pregnancy except in extreme situations when the motherís life is at risk or when thyroid surgery is required. These medications reduce placenta transfer of nutrients and may lead to decreased growth or decreased fetal thyroid production. Radioactive iodine also is not used in pregnancy since it not only can destroy motherís thyroid tissue, but will pass the placenta and destroy the fetusís thyroid. PTU is the most often used medication for hyperthyroidism in pregnancy and it must be used in the lowest possible dose to manage mother without decreasing fetal thyroid production. Growth of the fetus must be closely monitored through the pregnancy. Surgery to remove part of the thyroid is usually only done during pregnancy when the motherís hypothyroidism cannot be controlled with anti-thyroid medications. The safest time for surgery is in the second trimester, and the mother has to be properly prepared for the surgery and the fetus monitored for safety during the procedure.
Even when a mother is properly treated for hyperthyroidism, the baby is still at risk for the development of hyperthyroidism. Therefore, the baby will need to be monitored closely after birth for the disease. Discussion with the pediatrician or pediatric endocrinologist should be carried out so the baby can get the needed monitoring.
If you have hyperthyroidism and are on medication, discuss the risks and complications with a physician and plan a course of therapy prior to conception. The obstetrician should always be included in the discussion of care.