Hereditary factors and acquired genetic changes can increase your risk of developing an underactive thyroid gland, but hypothyroidism can develop for different reasons, and they may not always be genetic.

Hypothyroidism, also referred to as underactive thyroid, is a condition caused when your thyroid gland doesn’t produce enough hormones. Thyroid hormones, particularly thyroxine (T4) and triiodothyronine (T3), are important regulators of your metabolism.

Without enough thyroid hormones, your body’s functions slow down. You may experience symptoms like fatigue, weight gain, body aches, and cold intolerance. In rare cases, hypothyroidism can lead to myxedema coma, a potentially life threatening condition of organ failure.

There are many different reasons why the thyroid gland becomes underactive, and genetics can play a role at birth as well as throughout your lifetime.

Some, but not all, people living with hypothyroidism are born with it. When this happens, it’s known as congenital hypothyroidism.

Congenital hypothyroidism affects approximately 1 in 2,500 newborns. It occurs when the thyroid gland itself isn’t fully formed at birth or when fetal development leads to errors in the making of thyroid hormones.

The majority of congenital hypothyroid cases involve an underdeveloped thyroid gland. Without treatment, congenital hypothyroidism can affect growth and motor development and may cause intellectual challenges later in life.

Does this mean congenital hypothyroidism is inherited?

Being born with a congenital condition doesn’t always mean it was passed down from your parents. Congenital simply means it was present at birth and not acquired later.

There are many reasons why the thyroid gland might not develop properly in a fetus. It’s not always clear why it happens. Any disruption to fetal development during the formation of the thyroid gland could impact function.

Genetic factors are thought to account for approximately 2–5% of congenital hypothyroidism diagnoses, though prevalence can vary depending on the population.

Hypothyroidism is sometimes linked to environmental factors like exposure to tobacco smoke and certain viruses, but it can also have underlying genetic influences.

For some people, hypothyroidism is a complex mix of environmental and genetic factors.

Hypothyroidism genetic component

Genetic causes of hypothyroidism include those passed down to you from your parents, hereditable factors, and those related to acquired genetic mutations, which are changes to your DNA specific to certain cells rather than your entire genetic code.

Approximately 65% of your thyroid hormone production — adequate or inadequate — is attributed to genetics. Genes instruct your body on how to perform everyday functions that affect your thyroid directly and indirectly.

Some genetic components, like those found in congenital hypothyroidism, may disrupt thyroid gland formation or function, while others, like those associated with autoimmune thyroid disorders, may increase the risk of developing hypothyroidism later in life.

Some genetics passed down from your parents may even affect how well you respond to thyroid hormone replacement therapy in hypothyroidism.

A 2018 study, for example, found an inherited genetic mutation could alter how the body metabolizes thyroid hormone. For people living with this genetic variant, traditional treatments with the medication levothyroxine might not be as effective.

Hypothyroidism environmental component

Environmental influences in hypothyroidism are external factors and conditions that contribute to an underactive thyroid. They include elements in the environment around you, your lifestyle factors, and things you put into your body, like medications.

A 2021 literature review found multiple environmental factors negatively affected thyroid health, including:

  • smoking
  • alcohol consumption
  • high body mass index (BMI)
  • certain micronutrients in food
  • exercise levels
  • exposure to pollutants

Certain medications, surgery on the thyroid gland, and exposure to radiation are environmental factors that may also contribute to hypothyroidism.

Some subtypes of hypothyroidism may be more heavily influenced by environmental factors than others. According to a 2017 review, environmental factors account for approximately 20% of autoimmune hypothyroidism causes.

Autoimmune hypothyroidism conditions, the most common causes of hypothyroidism, are those in which your body’s immune system mistakenly attacks the tissues of the thyroid gland. The most frequently seen presentation is Hashimoto’s thyroiditis (HT).

Genetics can also play a role in your overall risk for developing autoimmune conditions, however. If you’ve been diagnosed with hypothyroidism, it’s possible genetic and environmental factors are involved, not just one or the other.

Autoimmune disease is responsible for most diagnoses of hypothyroidism.

Hashimoto’s disease, also known as Hashimoto’s thyroiditis, is the most common cause of hypothyroidism. It occurs when anti-thyroid antibodies in the immune system attack healthy thyroid tissue.

Other common causes of hypothyroidism include:

  • thyroiditis, inflammation of the thyroid
  • congenital hypothyroidism
  • radiation therapy
  • surgical removal of all or part of the thyroid
  • iodine imbalance
  • damage to the pituitary gland
  • certain medications, like lithium

If one or more of your family members have been diagnosed with congenital hypothyroidism, there’s as much as a 15-fold increased risk a child in your family will be born with congenital hypothyroidism.

This means the likelihood of inheriting hypothyroidism is 15 times higher compared to the baseline risk for the general population.

Some people may be able to prevent hypothyroidism based on their individual risk factors. Not all causes of hypothyroidism, like underlying genetics, can be modified, however.

Maintaining a healthy lifestyle, eating a balanced diet, avoiding radiation and pollutant exposure, and smoking cessation can all improve your individual risk.

Hypothyroidism is a condition that occurs when your thyroid gland is underactive. The exact underlying mechanisms aren’t clear, but genetics and environmental factors both likely play a role.

While it might not always be possible to prevent hypothyroidism, you can improve individual risk factors such as those related to lifestyle, substance use, and pollutant exposure.

Understanding your familial risk can help you and your doctor create a plan to monitor thyroid function as you grow older.