The information provided below is for readers based in the United States of America. Readers outside of the United States of America should seek the information from local sources.

What is hypothyroidism during pregnancy?

Changes in thyroid function are related closely to pregnancy. First of all, thyroid abnormalities can impact your fertility, making it difficult to get pregnant in the first place. Such abnormalities include hypothyroidism, meaning that thyroid activity is below normal; the thyroid gland is not releasing adequate amounts of the thyroid hormones triiodothyronine (T3) and thyroxine (T4), which decelerates your metabolism. If you do get pregnant, however, hypothyroidism impacts your pregnancy, but pregnancy impacts your hypothyroid condition. As pregnancy advances, there is an increased need for thyroid hormones, but there is a rise in pregnancy hormone, beta-human chorionic gonadotropin (ß-hCG). Peaking at the end of the first trimester, ß-hCG has a chemical structure similar to another hormone called thyroid-stimulating hormone (TSH), which comes from the pituitary gland in the brain and stimulates the thyroid when levels of T3 and T4 get too low. ß-hCG thus stimulates the thyroid as does the female hormone estrogen, which also rises during pregnancy. Consequently, if you are hypothyroid, but your thyroid is a little bit active, its possible that your condition might improve during pregnancy. However, due to the increased need for thyroid activity during pregnancy, its also possible that your condition will worsen or not change much at all.

One of the main causes of hypothyroidism is an autoimmune condition called Hashimoto thyroiditis, in which the immune system attacks the thyroid until it no longer functions adequately. You also can become hypothyroid if you over-respond to anti-thyroid drugs that are given to treat the opposite condition, hypERthyrodism, meaning an overactive thyroid. Hyperthyroidism also is often treated with iodine-131, which is absorbed selectively into the thyroid, where it releases gamma rays that have a destructive effect on thyroid tissue. The strategy in such cases is to deliver just enough iodine-131 to destroy an appropriate amount of the thyroid to leave the person euthyroid (having normal thyroid activity). However, it is challenging to control the destruction of thyroid tissue, and doctors err on the side of leaving the patient hypothyroid. Iodine-131 is also given to treat certain types of thyroid cancer, in which case the result is also hypothyroidism. Thyroid cancer and hyperthyroidism also are often treated with thyroidectomy partial or complete removal of the thyroid, which leads to hypothyroidism. Additionally, various head and neck cancers are treated with radiation therapy against which it is sometimes difficult to shield the thyroid, in which case hypothyroidism can result as well. Finally, there are some medications given for non-thyroid conditions that can decrease your thyroid function, one example being lithium, which is given for bipolar disorder.

How common is hypothyroidism during pregnancy?

Hypothyroidism has been reported with prevalence ranging from 1.5 to 4.4 percent of pregnant women. This makes hypothyroidism the most common type of thyroid disorder during pregnancy. Most of the cases are the result of Hashimoto disease.

How is hypothyroidism during pregnancy diagnosed?

To begin, your doctor will suspect hypothyroidism, if you suffer from hypothyroid symptoms, such as fatigue, sluggishness, muscle weakness, aches, and stiffness, constipation, joint pain and stiffness, dry, pale skin, puffy face, brittle nails, hair loss, enlarged tongue, depressed mood, memory difficulty, feeling that you are too cold, and weight gain, beyond the appropriate weight gain for your stage of pregnancy. Normally, you should gain 1 to 2 kilograms (2 to 5 pounds) during the first trimester, and then 0.5 kg (1 lb) each week.

To diagnose hypothyroidism, blood is drawn and tested for levels of TSH and free T4 (T4 hormone that is not bound to a carrier protein called Thyroid-binding globulin [TBG]). If TSH is elevated and free T4 is below normal, then your doctor will diagnose hypothyroidism, resulting from a problem in the thyroid. Less commonly, it is possible to have hypothyroidism that is secondary to a problem in the pituitary gland or other location in the brain, in which case free T4 would be low, but TSH would be normal or low. If you are hypothyroid, indicated by a high TSH and low free T4, you will be worked up for Hashimoto thyroiditis. Your blood will be tested for the presence of antibodies against an enzyme called thyroid peroxidase (TPO). The TPO antibody assay is not specific for Hashimoto thyroiditis (many people test positive for TPO antibodies without thyroid abnormalities), so the results must be interpreted in the context of your symptoms and signs, such as elevated TSH and low free T4.

In many women, laboratory values, especially elevated TSH, suggest hypothyroidism, even when they do not experience any hypothyroid symptoms. This is known as subclinical hypothyroidism (SCH), and it is present in an estimated 3 to 8 percent of the general population.

Does hypothyroidism cause problems during pregnancy?

Hypothyroidism decelerates your metabolism, leading to a range of symptoms, such as Hashimoto disease, which includes fatigue, feeling sluggish, weight gain (beyond what is expected in pregnancy), muscle weakness, muscle aches and stiffness, joint pain and stiffness, constipation, dry, pale skin, puffy face, hair loss, and brittle nails. If left untreated, the condition can deteriorate into an extreme form of hypothyroidism called myxedema, characterized by abnormally low body temperature (hypothermia) and slowing of multiple organs, and decreasing mental status, ultimately reaching a comatose stage, known as myxedema coma.

Does hypothyroidism cause problems for the baby?

Hypothyroidism is associated with an elevated risk of preterm birth and low-birth-weight (LBW), low Apgar score, and possibly small size for gestational age and intrauterine fetal death. When it comes to LBW and low Apgar score, research suggests increased risk, even in cases of SCH. If the hypothyroidism is due to Hashimoto thyroiditis, there is also an elevated risk of neonatal goiter, and thyrotoxicosis (excessively high levels of thyroid hormones).

What to consider about taking medications when you are pregnant or breastfeeding:

  • The risks to yourself and your baby if you do not treat the hypothyroidism
  • The risks and benefits of each medication you use when you are pregnant
  • The risks and benefits of each medication you use when you are breastfeeding

What should I know about using medication to treat hypothyroidism during pregnancy?

Hypothyroidism is treated with synthetic thyroid hormone called levothyroxine (L-T4) that you must take every day, which helps you and also decreases the risk that the baby will suffer harmful effects. Research shows that even when hypothyroidism is subclinical, taking L-T4 improves the outcome for the baby.

Who should NOT stop taking medication for hypothyroidism during pregnancy?

Most women with either clinical or subclinical hypothyroidism prior to pregnancy should continue to take their L-T4 after getting pregnant, whereas stopping the treatment puts the baby at elevated risk for LBW and other adverse outcomes.

What should I know about choosing a medicatio
n for my hypothyroidism during pregnancy?

It is important to stay in communication with your health care provider as the release of new studies over time can change the outlook on the role of specific medications during pregnancy.

You may find Pregistrys expert reports about the medications to treat this condition here. Additional information can also be found in the sources listed below.

What should I know about taking a medication for my hypothyroidism when I am breastfeeding?

It is safe to breastfeed while you are taking L-T4.

What alternative therapies besides medications can I use to treat my hypothyroidism during pregnancy?

There is no alternative to taking synthetic thyroid hormone for hypothyroidism. However, the future may bring new treatments in the area of bioregenerative medicine that would produce new thyroid tissue.

What can I do for myself and my baby when I have hypothyroidism during pregnancy?

Take your synthetic thyroid hormone and follow the instructions of your physician.

Resources for hypothyroidism in pregnancy:

For more information about hypothyroidism during and after pregnancy, contact http://www.womenshealth.gov/ (800-994-9662 [TDD: 888-220-5446]) or contact the following organizations:

Read the whole report
General information

It is very common for women to worry about having a miscarriage or giving birth to a child with a birth defect while they are pregnant. Many decisions that women make about their health during pregnancy are made with these concerns in mind.

For many women these concerns are very real. As many as 1 in 5 pregnancies end in a miscarriage, and 1 in 33 babies are born with a birth defect. These rates are considered the background population risk, which means they do not take into consideration anything about the health of the mom, the medications she is taking, or the family history of the mom or the baby’s dad. A number of different things can increase these risks, including taking certain medications during pregnancy.

It is known that most medications, including over-the-counter medications, taken during pregnancy do get passed on to the baby. Fortunately, most medicines are not harmful to the baby and can be safely taken during pregnancy. But there are some that are known to be harmful to a baby’s normal development and growth, especially when they are taken during certain times of the pregnancy. Because of this, it is important to talk with your doctor or midwife about any medications you are taking, ideally before you even try to get pregnant.

If a doctor other than the one caring for your pregnancy recommends that you start a new medicine while you are pregnant, it is important that you let them know you are pregnant.

If you do need to take a new medication while pregnant, it is important to discuss the possible risks the medicine may pose on your pregnancy with your doctor or midwife. They can help you understand the benefits and the risks of taking the medicine.

Ultimately, the decision to start, stop, or change medications during pregnancy is up to you to make, along with input from your doctor or midwife. If you do take medications during pregnancy, be sure to keep track of all the medications you are taking.

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