Thyroid Disease in Pregnancy: What to Know

Thyroid hormones are necessary for normal fetal development during the first 3 months. Because hypothyroidism and hyperthyroidism are possible, thyroid levels should be monitored before and throughout your pregnancy.

Written by Grazia Aleppo MD, FACE, FACP

The thyroid diseases—hyperthyroidism and hypothyroidism—are relatively common in pregnancy and important to treat. The thyroid is an organ located in the front of your neck that releases hormones that regulate your metabolism (the way your body uses energy), heart and nervous system, weight, body temperature, and many other processes in the body.

Thyroid hormones are particularly necessary to assure healthy fetal development of the brain and nervous system during the first three months of your pregnancy since the baby depends on your hormones, which are delivered through the placenta. At around 12 weeks, the thyroid gland in the fetus will begin to produce its own thyroid hormones.

There are 2 pregnancy-related hormones: estrogen and human chorionic gonadotropin (hCG)  that may cause your thyroid levels to rise. This may make it a bit harder to diagnose thyroid diseases that develop during pregnancy. However, your doctor will be on the look-out for symptoms that suggest the need for additional testing.

However, if you have pre-existing hyperthyroidism or hypothyroidism, you should expect more medical attention to keep these conditions in control while you are pregnant, especially for the first trimester. Occasionally, pregnancy may cause symptoms similar to hyperthyroidism; should you experience any uncomfortable or new symptoms, including palpitations, weight loss, or persistent vomiting, you should, of course, contact your physician.

Untreated thyroid diseases during pregnancy may lead to premature birth, preeclampsia (a severe increase in blood pressure), miscarriage, and low birth weight among other problems. Therefore, it is important to talk to your doctor if you have had a history of hypothyroidism or hyperthyroidism so you can be monitored before and during your pregnancy, and to be sure that your medication is properly adjusted, if necessary.  

Symptoms of Hyperthyroidism & Hypothyroidism While Pregnant

Thyroid hormones 

Hyperthyroidism
Symptoms of hyperthyroidism may mimic those of normal pregnancy, such as an increased heart rate, sensitivity to hot temperatures, and fatigue. Other symptoms of hyperthyroidism include the following:

Hypothyroidism
Symptoms of hypothyroidism, such as extreme tiredness and weight gain, may be easily confused with normal symptoms of pregnancy. Other symptoms may include:

Causes of Thyroid Disease in Pregnancy
Hyperthyroid disease—The most common cause of maternal hyperthyroidism during pregnancy is the autoimmune disorder Grave’s disease. In this disorder, the body makes an antibody (a protein produced by the body when it thinks a virus or bacteria is present) called thyroid-stimulating immunoglobulin (TSI) that causes the thyroid to overreact and make too much thyroid hormone.

Even if you've had radioactive iodine treatment or surgery to remove your thyroid, your body can still make the TSI antibody. If these levels rise too high, TSI will travel through your blood to the developing fetus, which may cause its thyroid to begin to produce more hormone than it needs. So long as your doctor is checking your thyroid levels, both you and your baby will get the care needed to keep any problems in check.

Hypothyroid disease—The most common cause of hypothyroidism is the autoimmune disorder known as Hashimoto’s thyroiditis. In this condition, the body mistakenly attacks the cells of the thyroid gland, leaving the thyroid without enough cells and enzymes to make enough thyroid hormone to meet the body's needs. 

Diagnosis of Thyroid Disease in Pregnancy
Hyperthyroidism and hypothyroidism in pregnancy are diagnosed based on symptoms, physical exam, and blood tests to measure levels of thyroid-stimulating hormone (TSH) and thyroid hormones T4, and for hyperthyroidism also T3

Treatment of Thyroid Disease in Pregnancy
For women who require treatment for hyperthyroidism, an antithyroid medication that blocks production of thyroid hormones is used. This medication—propylthiouracil (PTU)—is usually given during the first trimester, and — if necessary, methimazole can be used, after the first trimester. In rare cases in which women do not respond to these medications or have side effects from the therapies, surgery to remove part of the thyroid may be necessary. Hyperthyroidism may get worse in the first 3 months after you give birth, and your doctor may need to increase the dose of medication. 

Hypothyroidism is treated with a synthetic (man-made) hormone called levothyroxine, which is similar to the hormone T4 made by the thyroid. Your doctor will adjust the dose of your levothyroxine at diagnosis of pregnancy and will continue to monitor your thyroid function tests every 4-6 weeks during pregnancy. If you have hypothyroidism and are taking levothyroxine, it is important to notify your doctor as soon as you know you are pregnant, so that the dose of levothyroxine can be increased accordingly to accommodate the increase in thyroid hormone replacement required during pregnancy. Because the iron and calcium in prenatal vitamins may block the absorption of thyroid hormone in your body, you should not take your prenatal vitamin within 3-4 hours of taking levothyroxine. 

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