Causes and Risk Factors of Hypothyroidism

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hypothyroidism causes
© Verywell, 2018

Hypothyroidism may develop for a number of different reasons, the most common of which is an autoimmune attack on the thyroid gland, called Hashimoto's thyroiditis. Hypothyroidism may also be the result of taking a medication like lithium, your genetics, or an underlying pituitary gland problem.

Understanding the "why" behind a diagnosis of hypothyroidism is critical to moving forward with a proper treatment plan.

While some people may require lifelong thyroid hormone replacement, others may have a short-lived case of hypothyroidism (for example, postpartum thyroiditis), need to stop taking a medication, or require further diagnostic tests like imaging of the pituitary gland.  

Common Causes

Hashimoto's thyroiditis is the leading cause of hypothyroidism in the United States. In Hashimoto's, antibodies react against proteins in your thyroid gland, causing gradual destruction of the gland itself,  rendering it unable to produce the thyroid hormones your body needs.

Hashimoto's thyroiditis is more common in women, and while it may occur at any age, it's more common as people get older. For women, Hashimoto's often develops during pregnancy, after delivery, or around the time of menopause. 

Other causes of hypothyroidism include:

Surgery

People with hyperthyroidism, thyroid nodules, or thyroid cancer may need to have surgery.

If all of the thyroid gland is surgically removed, a person will be hypothyroid and require lifelong thyroid hormone replacement medications. If only part of the thyroid gland is removed, there is a good chance that it will still be able to make sufficient thyroid hormone. 

Radiation

Instead of thyroid surgery or antithyroid medication, some people with hyperthyroidism are treated with radioactive iodine, which will destroy the thyroid gland, rendering a person hypothyroid.

People who undergo radiation treatment for Hodgkin's lymphoma or head and neck cancer are also at risk for developing hypothyroidism. 

Thyroiditis

Thyroiditis describes thyroid gland inflammation and is a general term for a variety of thyroid conditions. Hashimoto's disease (as mentioned above) is the most common type of thyroiditis and is caused by an autoimmune attack.

Another example of thyroiditis is subacute thyroiditis (also called de Quervain's thyroiditis), which is believed to be caused by a virus. With this type of thyroiditis, a person experiences hyperthyroidism followed by hypothyroidism, in addition to a tender thyroid gland. 

Certain Drugs

Certain medications may trigger hypothyroidism. These medications include:

  • Lithium
  • Amiodarone
  • Thionamides (antithyroid drugs)
  • Interferon-alpha
  • Interleukin-2
  • Certain cancer drugs (tyrosine kinase inhibitors and checkpoint inhibitor immunotherapies)

Iodine Excess or Deficiency

Too much iodine (for example, from dietary supplements that contain kelp) can cause or worsen hypothyroidism. In addition, a deficiency of iodine, which is seen in some people in underdeveloped countries, may cause hypothyroidism. Iodine is necessary for the production of thyroid hormone and is found in foods, like dairy products, chicken, beef, pork, fish, and iodized salt.

Congenital Hypothyroidism

Some babies are born without a thyroid gland or with only a partial thyroid gland. Since there is no thyroid gland (or not enough) to produce thyroid hormone, hypothyroidism develops, which is serious and requires treatment with a thyroid hormone pill.

Pituitary Gland Problem

The pituitary gland is located in the brain and stimulates other glands within the body, like the thyroid gland, to release hormone. If the pituitary gland is damaged by a brain tumor, radiation, or brain surgery, it may not function well enough to signal the thyroid gland. This can then result in it becoming underactive.

This type of hypothyroidism is called central or secondary hypothyroidism. 

Infiltrative Diseases

Rarely, certain diseases, like hemochromatosis, can deposit abnormal substances (iron, in the case of hemochromatosis) in your pituitary gland, causing central hypothyroidism, or less commonly, your thyroid gland, causing primary hypothyroidism. 

Besides hemochromatosis, sarcoidosis can cause granuloma deposition in the thyroid gland. There is also a rare condition called fibrous thyroiditis (or Riedel's thyroiditis), in which fibrotic tissue replaces normal thyroid tissue. 

Genetics

Your DNA plays a role when it comes to developing autoimmune hypothyroidism, and this has been supported by a number of studies.

For instance, one German study found a 32-fold increased risk for developing Hashimoto's thyroiditis in children and a 21-fold increased risk in siblings of people with Hashimoto's thyroiditis.  

When looking at the specific genes linked to Hashimoto's, scientists have found mutations in the genes for human leukocyte antigen (HLA), T-cell receptors, and other molecules involved in the immune system. 

To further support the role of genes in developing Hashimoto's thyroiditis, people with Turner syndrome and Down syndrome (both of which are genetic in origin) have a higher than expected rate of autoimmune thyroid disease, especially Hashimoto's thyroiditis. 

All said, though, it's important to remember that your genes are but one factor that helps predict your risk of developing hypothyroidism. There are many other factors that come into play, like pregnancy or taking certain medications.

In the end, it's the combination of genes and an environmental trigger that predict a person's unique risk for developing hypothyroidism. 

Risk Factors

Factors that increase a person's risk of developing hypothyroidism include:

  • Being female
  • Being of an older age
  • Being Caucasian or Asian
  • Having a family history of Hashimoto's thyroiditis or another autoimmune disease
  • Having a personal history of an autoimmune disease (for example, adrenal insufficiency, rheumatoid arthritis, or type 1 diabetes)
  • Being pregnant or postpartum
  • Too much or too little iodine consumption
  • Treated with radioactive iodine
  • Received radiation to the neck or upper chest
  • Underwent thyroid surgery
  • Treated with certain medications (for example, lithium for bipolar disorder)

Evolving Risk Factors

Interestingly, research suggests that selenium deficiency may be linked to developing Hashimoto's thyroiditis and hypothyroidism. Moreover, having underlying headache disorders, like migraines, has been found to be associated with an increased risk of hypothyroidism, especially in obese women. 

It's still unclear precisely how smoking affects the thyroid gland, although it's likely complex. While some studies suggest that cigarette smoking increases the risk of hypothyroidism in people with Hashimoto's thyroiditis, other research suggests that smoking is actually linked to a lower prevalence of hypothyroidism and a higher prevalence of hyperthyroidism. 

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Dittmar M, Libich C, Brenzel T, Kahaly GJ. Increased familial clustering of autoimmune thyroid disease. Horm Metab Res. 2011 Mar;43(3):200-4. 

Garber JR et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012 Nov-Dec;18(6):988-1028.

Martin AT et al. Headache disorders may be a risk factor for the development of new onset hypothyroidism. Headache. 2017 Jan;57(1):21-30. 

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