Hyperthyroidism and Pregnancy

Medical Author: Ruchi Mathur, M.D.
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR

At first when I started to write on this topic, I thought of my rotation through Obstetrics as a medical student. I particularly remember one patient during my first week who had Graves' disease and presented to the hospital in her 6th month of pregnancy with uncontrolled hyperthyroidism (overactive thyroid). The attending doctor's face was stern and serious as he went around the table and asked us why we should be concerned. This article will discuss hyperthyroidism in pregnancy and its effects on both the mother and child.

During pregnancy, the hormone HCG (human chorionic gonadotropin) is produced. HCG is the hormone that "pregnancy tests" detect. HCG increases to a peak at around 12 weeks. It has mild thyroid stimulating effects and, as a result, can cause some symptoms of hyperthyroidism. HCG is in part responsible for the nausea during the first trimester. In situations of multiple pregnancies (twins, triplets) HCG levels are even higher, and symptoms can be more pronounced. Temporary subclinical (no apparent symptoms) hyperthyroidism occurs in 10-20% of normal pregnant women during this period, and these women typically do not require treatment.

Hyperemesis gravidarum is a syndrome of nausea and vomiting that is associated with weight loss of more than 5% during early pregnancy. It is associated with high levels of HCG. Sixty percent of these women have either subclinical (no apparent symptoms) or mildly clinical (mild symptoms) hyperthyroidism, which also usually resolves after the first trimester.

There are other rare conditions associated with abnormal pregnancies that result in hyperthyroidism that are beyond the scope of this discussion.

While the conditions mentioned above are self-limited and associated with changes in the first trimester of pregnancy, women who have hyperthyroidism and become pregnant present another issue.

Pregnancies complicated by uncontrolled hyperthyroidism may result in higher incidences of:

  • Spontaneous abortion (miscarriage)
  • Preterm labor
  • Low birth-weight babies
  • Stillbirths
  • Complications of pregnancy, including pre-eclampsia (a condition associated with hypertension, low blood platelet count, protein in the urine and mental changes) and heart failure.

Although any form of uncontrolled hyperthyroidism can complicate pregnancies, the most common form is Graves' disease.

The diagnosis of hyperthyroidism in pregnancy can be complicated since some of the blood tests used for the diagnosis are altered because of the pregnancy. The diagnosis is based on high levels of thyroid hormones, T3 and T4, and a low level of thyroid stimulating hormone (TSH).

The treatment of hyperthyroidism in pregnancy is limited because the safety of the baby must also be considered. Usually, drugs such as propylthiouracil (PTU) and methimazole (MMI) are used. While both of these drugs do cross the placenta and can enter the baby's system, treatment is still preferred because of the poor outcomes associated with not treating these women. PTU is preferred because MMI has been associated with a rare scalp condition in the fetus known as "aplasia cutis."

Medications to slow the mother's heart rate down may also be necessary. The class of drugs recommended is called beta-blockers (metoprolol, propranolol). While these drugs are not thought to be dangerous to the fetus (teratogenic), there have been associations with growth retardation. Low blood sugars at birth and some respiratory problems have also been reported.

Since radiation therapy is not safe for the baby in pregnancy, surgery may be the only other option for women who cannot tolerate medicine treatments. Surgery itself is associated with an increased risk of premature delivery and spontaneous abortion (miscarriage).

Of the babies born to women with Graves' disease, about 1% will have hyperthyroidism at birth. The cause is the transfer of antibodies that stimulate the TSH receptor across the placenta to the baby. These antibodies then stimulate the baby's thyroid gland. These antibodies should be measured in the mother during her second trimester of pregnancy, because values that are greater than five times normal have been associated with hyperthyroidism in the baby at birth. Before the baby is born, a high fetal heart rate (greater than 160 beats/min), a goiter in the fetus noted during ultrasound, poor growth, and bony abnormalities are indications of hyperthyroidism in the baby. If these are noticed, the mother may be given medications (PTU or MMI) to help treat the fetus. At birth, hyperthyroidism may be detected by a blood test. On physical examination, a low birth weight, enlarged liver or spleen, and abnormalities in the shape of the skull (smallish head, triangular face) can be clues to hyperthyroidism in the newborn.



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