Thyroid Disorders and Pregnancy | The Children's Hospital of Philadelphia

Pediatric Thyroid Center

Thyroid Disorders and Pregnancy

Before you begin reading about thyroid disorders and pregnancy, please read a description of the thyroid gland for a basic understanding of its structure and function.

Hypothyroidism and pregnancy

How does hypothyroidism affect the fetus?

During the first few months of pregnancy, the fetus relies on the mother for thyroid hormones. Thyroid hormones play an essential part in normal brain development. Deprivation of the maternal thyroid hormone due to hypothyroidism can have irreversible effects on the fetus. Early studies found that children born to mothers with hypothyroidism during pregnancy had lower IQ and impaired psychomotor (mental and motor) development. If properly controlled, often by increasing the amount of thyroid hormone, women with hypothyroidism can have healthy, unaffected babies.

For patients with chronic lymphocytic thyroiditis (CLT), also called Hashimoto’s thyroiditis, there is some evidence to suggest an increased risk of pregnancy loss. CLT is a condition in which the immune system attacks the thyroid gland leading to damage and decreased thyroid function. Some studies have shown a higher rate of stillbirth and miscarriage in pregnant women who have CLT, while others found no increase. 

Current recommendations are to verbally screen all women at the initial prenatal visit for any history of thyroid dysfunction or thyroid hormone medication. Laboratory screening of thyroid functions and/or thyroid antibodies should be considered for women at high risk of hypothyroidism. Detection and treatment of maternal hypothyroidism early in pregnancy may prevent the harmful effects of maternal hypothyroidism on the fetus. For women on thyroid hormone prior to conception, thyroid function testing should be performed regularly throughout pregnancy as it is very likely that the thyroid hormone dose will need to be increased. Women are encouraged to ask their primary care providers for further information and clarification on this important topic.

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Hyperthyroidism and pregnancy

How does pregnancy affect hyperthyroidism?

It does not appear that pregnancy worsens hyperthyroidism or complicates treatment in women with this condition.

Healthy thyroid glands function normally during pregnancy. Less than 1 percent of women have excessive thyroid function during pregnancy.

How does hyperthyroidism affect pregnancy?

Uncontrolled hyperthyroidism has many effects. It may lead to preterm birth (before 37 weeks of pregnancy) and low birth weight for the baby. Some studies have shown an increase in pregnancy-induced hypertension (high blood pressure of pregnancy) in women with hyperthyroidism.

A severe, life-threatening form of hyperthyroidism, called thyroid storm, may complicate pregnancy. This is a condition in which there are extremely high levels of thyroid hormone that can cause high fever, dehydration, diarrhea, rapid and irregular heart rate, shock and death, if not treated.

It is always best to plan for pregnancy and to consult with your physician to ensure your thyroid status and treatment are optimized prior to becoming pregnant and monitored throughout your pregnancy. However, if this does not happen and you find out you are pregnant, you should contact your physician immediately to arrange for increased testing of your thyroid functions and a potential change in your medication.

Management of hyperthyroidism during pregnancy

Treatment for hyperthyroidism is very specific for each patient. The goal of treatment is to maintain normal levels of thyroid hormone. Treatment may include:

The most commonly prescribed anti-thyroid medication, called Methimazole or Tapazole (MMI), may be associated with birth defects. Current recommendations are to stop MMI during the first trimester and to use Propylthiouracil (PTU) with a switch back to MMI at the start of the second trimester and for the remainder of the pregnancy. Use of radioactive iodine, in the form of a pill or liquid, damages thyroid cells and is not safe during pregnancy or during lactation (breastfeeding). For women who are given radioiodine ablation therapy to treat Graves’ disease, it is recommended that they wait at least one year after radioiodine treatment to become pregnant. Your healthcare provider will discuss the risks and benefits of treatment with you.

Women with hyperthyroidism can increase their chances for a healthy pregnancy by getting early prenatal care and working with their healthcare providers in the management of their disease.

Are there any risks to a baby after birth?

Neonatal Graves’ disease occurs in about 1 percent of babies born to mothers with active Graves’ disease or a history of the disease. Babies may be severely affected requiring hospitalization and intensive care support. In its most severe form, hyperthyroidism in the newborn can be fatal. In less severe forms, and with good control, the consequences of Graves’ disease on the baby are usually temporary. However, even under the best of circumstances there may be permanent consequences of maternal Graves’ disease on the baby.

The cause of Graves’ disease in the newborn is the crossing of the mom’s antibodies through the placenta to the baby. Even for women who were definitively treated for their Graves’ disease, the maternal antibodies may be present for years afterwards and continue to be a potential risk to the baby. The anti-thyroid medications (MMI or PTU) that the mom takes may also have temporary or permanent effects on the baby. Because of these concerns, it is extremely important to let your physician know if you have Graves’ disease or a history of Graves’ disease to ensure both you and your baby are followed more closely.

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