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Hypothyroidism, a condition where the thyroid gland doesn’t produce enough thyroid hormones, is a common concern among women. In fact, one in eight women will develop a thyroid condition at some point in her life. 

Pregnancy is one of the most common times when hypothyroidism is diagnosed in women. When it’s not properly treated, an underactive thyroid in an expectant mother can have adverse effects on the developing baby. These issues raise some common questions. Does a mother’s hypothyroidism get “passed down” to her newborn? What are the risk factors for hypothyroidism at birth, also known as congenital hypothyroidism? We’ll explore the answers in this article.

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The health of a pregnant person’s thyroid significantly impacts the health and development of the fetus in utero. The fetus depends on the mother’s thyroxine (T4) -- the storage thyroid hormone -- for brain development during the first 18 to 20 weeks in utero.

After that, the fetus starts to make their own thyroid hormone. However, the fetus still depends on the mother’s intake of iodine. Without enough dietary iodine, the fetus can’t produce thyroid hormones.

Maternal hypothyroidism can occur during pregnancy or in women with known hypothyroidism. In the United States, maternal hypothyroidism is often due to Hashimoto’s thyroiditis, a thyroid autoimmune disorder.

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A failure to diagnose or adequately treat hypothyroidism during pregnancy can increase the risk of various complications associated with untreated maternal hypothyroidism. These complications include: 

• Preeclampsia
• Pre-term birth
• Low birth weight
• Respiratory distress in the newborn
• Muscle pain
• Placenta abnormalities
• Increased risk of miscarriage
• Excessive postpartum bleeding

The severity, onset, and duration of maternal hypothyroidism influence the brain development of the fetus. Mild maternal hypothyroidism can affect fetal brain development and can be caused by:

• Mild iodine deficiency
• Hashimoto’s thyroiditis
• Too little thyroid medication

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The primary treatment for maternal hypothyroidism is taking thyroid hormone replacement medication. These medications normalize thyroid hormone levels. 

Pregnant people with hypothyroidism usually have their thyroid function tested more often. This is because most will need dose adjustments of their thyroid medication due to the body’s increased demand for thyroid hormone.

Keep in mind that prenatal vitamins are strongly recommended for everyone during pregnancy. But, suppose you take a thyroid medication like levothyroxine. In that case, you will want to take your prenatal supplements and medications separately. Certain minerals in the multivitamin, like iron and calcium, can affect the absorption of thyroid medication. Reduced absorption of your thyroid medication can lead to low thyroid hormone levels.

Iodine requirements

Iodine, a trace element, is required for the thyroid to produce thyroid hormones. Here’s how.

Thyroid-stimulating hormone (TSH) secretion from the pituitary increases the thyroid’s iodine uptake. Iodine stimulates the production and release of thyroid hormones T4 and T3 (triiodothyronine).

During pregnancy and lactation, the body’s need for iodine increases. The recommended intake goes up to 220 mcg and 290 mcg, respectively. Our bodies don’t make iodine, so we must get it from foods or supplements. Starting iodine supplements before or early in pregnancy may raise the amount of maternal T4 available for the fetus.

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Not necessarily.

Now, suppose a pregnant person has low thyroid hormone levels. In that case, the baby may be born with irreversible symptoms, including ones related to mental and physical development. This doesn’t mean the baby has hypothyroidism - it means they are showing side effects of maternal hypothyroidism. However, there may be a genetic link to hypothyroidism that isn’t fully understood.

The genetics of hypothyroidism is complex. Often, people with a thyroid condition report that someone in their family also has a thyroid disorder. Having a family member with a thyroid condition does put you or your child at a higher risk of developing hypothyroidism.

The most common cause of hypothyroidism is Hashimoto’s. In this autoimmune disease, the immune system mistakenly attacks the thyroid gland. This results in chronic inflammation. Chronic inflammation damages thyroid tissue, leaving it unable to make enough thyroid hormone to support the body’s needs.

In cases where an autoimmune process causes hypothyroidism, there is likely a genetic component. This doesn’t mean your baby will automatically develop hypothyroidism in their lifetime. Usually, a combination of genetics and environmental factors triggers the onset of Hashimoto’s.

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Babies born with an underactive or absent thyroid gland have what is called congenital hypothyroidism. Congenital hypothyroidism is rare, occurring in 1 in 2500 to 3000 newborns.

In this situation, the thyroid doesn’t form correctly during pregnancy. It can result in a baby having no thyroid gland at all, a small, partially developed one, a gland located in an abnormal place in the neck, or a fully developed gland that can’t make normal amounts of thyroid hormone. Congenital hypothyroidism is usually the result of a genetic problem.

The exact reason this happens is often unknown, but in some cases, it is genetic. However, it’s important to note that congenital hypothyroidism is usually not inherited through families. This means if one child is affected, it is unlikely that other children you may have in the future will suffer from the same condition.

If untreated, congenital hypothyroidism can lead to severe problems such as mental disability, growth delays, or loss of hearing. Therefore, early diagnosis and treatment are crucial to prevent these complications.

Certain factors can increase the risk of hypothyroidism in newborns (neonatal hypothyroidism). For instance, if a mother’s diet is low in iodine during pregnancy, the child may have low thyroid hormone levels at birth. Additionally, certain medications taken during pregnancy can lead to hypothyroidism in a newborn, but it’s usually temporary.

Next, we’ll focus on the two most common causes of hypothyroidism and the role of genes in the development of an underactive thyroid in newborns.

Thyroid dysgenesis

Thyroid dysgenesis (TD) accounts for 65% of congenital hypothyroidism cases. Those with TD either have no thyroid present or one that is severely underdeveloped. And no thyroid means no thyroid hormone production, leading to hypothyroidism.

Roughly 5 to 10% of TD cases have a genetic cause. In these cases, genetic mutations are found in genes linked to thyroid development. TD follows a dominant inheritance pattern, meaning only one copy of the abnormal gene needs to be passed down from a single parent.

Thyroid dyshormonogenesis

About 35% of congenital hypothyroidism cases are due to thyroid dyshormonogenesis (DH). In these cases, the baby has a thyroid, often of standard size or enlarged. But, the thyroid can’t make and secrete thyroid hormones, resulting in hypothyroidism.

DH is commonly caused by a defect in how the thyroid absorbs or utilizes iodine. As we know, iodine is essential for thyroid production. Most of these defects in iodine use are from alterations in genes. Research shows that DH is autosomal recessive. This means there must be two copies of an abnormal gene - one passed down from each parent - for DH to develop. Autosomal recessive disorders aren’t seen in every generation, so you may likely not have multiple family members with DH.

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The hypothalamus-pituitary-thyroid (HPT) axis regulates thyroid hormone production. In utero, the HPT axis starts functioning after the first trimester. Yet, it is not complete until term gestation. Because of this, infants born too early may have an immature hypothalamus-pituitary-thyroid axis. As a result, they often have dysregulation of thyroid hormone levels.

Premature infants often have low thyroid levels in the first few weeks of life. Sometimes, low T4 levels are temporary and will increase independently, unlike high levels of TSH, which can be temporary or permanent. And permanently high TSH levels may need treatment with thyroid medication.

Pre-term infants are at a greater risk for hypothyroidism. But whether hypothyroidism is transient or permanent is unpredictable. The need for thyroid hormone medication in premature infants remains unclear.

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The symptoms of congenital hypothyroidism may not be evident in the first weeks of life. Babies generally don’t show obvious signs or symptoms of low thyroid hormone levels. Because of this, most countries, including the United States, proactively test babies at birth for hypothyroidism.

Signs and symptoms of congenital hypothyroidism include the following:

• Jaundice
• Poor muscle tone
• Feeding challenges
• Poor or slow growth
• Puffy face
• Swelling around the eyes
• Larger fontanelles or “soft spots”
• Excessive sleeping
• Large, swollen tongue
• Constipation
• Cool pale skin
• A large belly with the navel sticking out 

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Neonatal screening programs started in the 1970s. Since then, there has been a significant improvement in developmental outcomes for children born with congenital hypothyroidism.

Testing

Hospitals in the United States screen babies for hypothyroidism at birth by collecting a blood sample from the baby’s heel. This heel stick measures the level of T4 and TSH. A low T4 level with an elevated TSH suggests congenital hypothyroidism.

If test results show an underactive thyroid, your baby’s healthcare provider will likely order another blood sample, this time from a vein. This second hypothyroidism test confirms the diagnosis before beginning treatment.

Treatment

Treatment is usually the same as hypothyroidism in adults. Babies with congenital hypothyroidism will need to take a daily dose of thyroid hormone replacement medication. This medication helps restore thyroid hormone levels. By doing so, the baby can have age-appropriate growth and development.

As the child gets older, thyroid hormone requirements change. Because of this, it is essential to schedule regular checkups with your child’s pediatric endocrinologist.

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While a mother’s hypothyroidism can potentially affect her baby’s health, it doesn’t directly cause hypothyroidism in the baby. However, certain genetic factors and environmental conditions during pregnancy can increase the risk of the baby developing congenital hypothyroidism. It’s crucial for expecting mothers with hypothyroidism to work closely with their healthcare providers to manage their condition, monitor the baby’s health, and ensure normal growth and development.

Having hypothyroidism does not guarantee that your baby will also have hypothyroidism. Indeed, there is still speculation about the role genes play in hypothyroidism.

Uncontrolled maternal hypothyroidism during pregnancy can, however, pass complications onto the child. With good prenatal care and thyroid medication, most babies are unaffected by hypothyroidism.

If you are pregnant and need to test your thyroid function frequently, consider ordering Paloma’s Complete Thyroid testing kit. Our kit measures your TSH and T4 levels, among other thyroid biomarkers. These levels help your provider adjust the dose of your thyroid medication during your pregnancy. A bonus: you can skip the lab visit and test when it is most convenient for you! Order your first kit today.

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