How Can Your Thyroid Affect Pregnancy?

The thyroid gland plays a crucial role during pregnancy in regulating the production of thyroid hormones. While many women experience pregnancy without any thyroid issues, other women will encounter a malfunctioning thyroid that, when not treated, can lead to complications for both woman and child. 

We learn from Mitchell West, M.D., an obstetrician/gynecologist (OBGYN) specializing in women’s health at INTEGRIS Health Medical Group Women's Baptist in Oklahoma City, about why thyroid issues happen, what complexities can develop and how to treat these conditions.

What are thyroid hormones?

The thyroid is a butterfly-shaped gland in your neck that produces hormones to regulate metabolism, growth and development.

The thyroid produces two main hormones called thyroxine (T4) and triiodothyronine (T3). The mineral iodine and the amino acid tyrosine help create these hormones. The thyroid-stimulating hormone (TSH) produced by the pituitary gland stimulates the secretion of T4 and T3.

• Thyroxine (T4): Thyroxine is the main thyroid hormone produced (accounts for 80 percent of the total thyroid hormone) but it doesn’t have as much of an effect on metabolism as T3. Once it reaches the bloodstream, T4 converts to T3 through a process called deiodination (the removal of iodine atoms).
• Triiodothyronine (T3): Triiodothyronine is more potent and has more of an impact than T4 – despite being produced in smaller quantities – because of its greater biological activity compared to thyroxine.

The thyroid gland also produces calcitonin, which regulates calcium and phosphate levels. Since calcitonin serves a different function, it isn’t grouped into the same “thyroid hormone” name as T4 and T3.

Here is an overview of the functions thyroid hormones assist with:

• Metabolism
• Heart rate
• Digestion
• Brain development
• Bone maintenance

The role of the thyroid during pregnancy

As soon as pregnancy begins, thyroid hormones kick into gear for two primary reasons – to aid in development of the fetus and to assist with the overall health of the mother. This need for an increased demand comes with an increased supply of thyroid hormones.

For the fetus, thyroid hormones help the brain to develop, bones to grow and metabolism to occur. For the mother, thyroid hormones also help regulate metabolism and energy levels needed to support a healthy pregnancy.

The fetus relies on maternal thyroid hormone throughout the pregnancy. The thyroid gland develops early in the first trimester and is able to produce functional thyroid hormones by 10-12 weeks. However, fetal production of thyroid hormone is not sufficient on its own. Although fetal production of thyroid hormone continues to rise well into the third trimester, the maternal thyroid hormones continue to support the fetus until near delivery.

Since thyroid function is so critical for both mother and child, it's important (if you have a past history of thyroid disorder) to have thyroid function monitored regularly to ensure thyroid hormone levels remain within range.

Your OBGYN can monitor thyroid function with a simple blood draw that measures thyroid-stimulating hormone (TSH). Typically, TSH falls between 0.5 and 5.0 milliunits per liter (mU/L). High TSH levels indicate an underactive thyroid (when thyroid hormones are low, the body releases more TSH to compensate), while low TSH levels indicate an overactive thyroid (when thyroid hormones are high, the body releases less TSH in an attempt to balance levels out).

Why can thyroid problems develop during pregnancy?

In many women, thyroid hormones work normally during a pregnancy. However, the thyroid can produce not enough or too many hormones, which can lead to problems for you and your child. Thyroid problems can start or worsen during pregnancy.

Hormonal changes during pregnancy are common, because the body is hard at work to support the necessary requirements needed to help a baby grow. In other words, the more the body changes, the more imbalances are likely to occur. For example, the hormone human chorionic gonadotropin (hCG) increases during the first trimester and can cause nausea and vomiting.

hCG and estrogen have a stimulatory impact on thyroid hormones and cause the thyroid to enlarge during pregnancy.

Thyroid problems can occur as early as the first trimester, which can cause complications since the fetus relies on a mother’s thyroid hormones during this time. Subclinical hypothyroidism, an underactive thyroid that occurs without obvious symptoms, is the most common type seen during pregnancy. However, subclinical hypothyroidism generally does not require treatment.

Hyperthyroidism in pregnancy

Hyperthyroidism occurs when the thyroid produces too many hormones. Graves’ disease, an autoimmune disease in which the immune system makes antibodies that cause the thyroid to produce excess T4 and T3, is usually the biggest culprit.

When symptoms do occur, the most common issues include anxiety, sweating, increased appetite, nervousness and a goiter on the neck. People with Graves’ disease are more at risk of developing thyroid storm, a rare but life-threatening complication that involves severe symptoms such as a high fever, rapid heartbeat, dehydration, diarrhea and shock.

Hypothyroidism in pregnancy

Hypothyroidism occurs when the thyroid doesn’t produce enough hormones. Hashimoto's thyroiditis, an autoimmune condition in which the immune system mistakenly attacks the thyroid gland, is usually the most common cause of low thyroid hormones during pregnancy.

When the body is unable to produce enough thyroid hormones, women may experience fatigue, lethargy, depression and constipation.

"Thyroid disease is common during pregnancy and, as a result, is commonly managed by OBGYNs throughout the course of pregnancy. If you have a history of a thyroid disorder, it is important to seek early prenatal care so that your OBGYN can check your thyroid levels and adjust medication accordingly. If identified early and managed actively throughout, pregnancy in women with thyroid disorders is most often successful and without major problems," says West.

Although rare, undiagnosed hypothyroidism in younger pregnant women can sometimes lead to severe hypothyroidism, a condition known as myxedema coma. This life-threatening condition can cause body functions to slow down to critical levels, including organ failure.

Thyroid disorders and pregnancy outcomes

Like many other diseases and disorders that occur during pregnancy, thyroid issues can increase your risk of miscarriage, preterm labor, preeclampsia and your baby being born at a low birth weight.

Specific side effects of hyperthyroidism during pregnancy include:

• Preterm birth
• Low birth weight
• Blood pressure disorders of pregnancy
• Fetal thyroid dysfunction
• Gestational high blood pressure
• Maternal heart problems
• Thyroid storm
• Miscarriage

Specific side effects of hypothyroidism during pregnancy include:

• Poor brain development in the baby
• Miscarriage
• Maternal anemia
• Maternal diabetes
• Maternal high blood pressure
• Myopathy (muscle pain)
• Preterm birth
• Low birth weight

Antithyroid medicines and synthetic hormones during pregnancy

While insufficient thyroid hormones during pregnancy can be a scary development for you and your child, you can still have a healthy pregnancy if the irregularities are treated.

The most common treatments for thyroid issues during pregnancy include the use of antithyroid medicines for hyperthyroidism and levothyroxine for hypothyroidism. 

Antithyroid medicines: These drugs work by blocking the action of an enzyme called thyroid peroxidase, which inhibits the production of thyroid hormones. Methimazole and propylthiouracil are two medications prescribed by doctors. The dose prescribed is a delicate balance between providing enough medication to treat an overactive thyroid while also avoiding the fetus not receiving enough thyroid hormone. As a result, a low dose is typically administered.

Levothyroxine: This drug is a synthetic version of thyroxine (T4). Levothyroxine mimics the function of naturally occurring thyroxine and allows your baby to receive T4 through the placenta. It also converts to triiodothyronine (T3) to help maintain metabolic activities during the pregnancy.

In rare cases, surgery may be necessary to treat thyroid issues when medications don’t work. A procedure called thyroidectomy, in which the thyroid gland is removed, can help treat hyperthyroidism. 

If you experience thyroid issues during your pregnancy, consult with your OBGYN to monitor thyroid hormone levels. For management of these issues, an endocrinologist can prescribe medications or additional treatment options to control an overactive or underactive thyroid.