Causes and Risk Factors of Hypothyroidism

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Hypothyroidism may develop for a number of different reasons, from an autoimmune attack on the thyroid gland (called Hashimoto's thyroiditis) to taking a medication like lithium. Hypothyroidism may also be the first sign of an underlying pituitary gland problem. Understanding the "why" behind a diagnosis of hypothyroidism is critical to moving forward with a treatment plan. While some people may require lifelong thyroid hormone replacement, as in majority of people with Hashimoto's thyroiditis, others may simply need to stop taking a certain medication, have a short-lived case of hypothyroidism (for example, postpartum thyroiditis), or require further diagnostic tests like imaging of the pituitary gland.

  

Common Cause

Hashimoto's thyroiditis is the leading cause of hypothyroidism in the United States.

Hashimoto's Thyroiditis

Hashimoto's thyroiditis, also called autoimmune thyroiditis, or chronic lymphocytic thyroiditis, is an autoimmune disease that affects your thyroid gland. In Hashimoto's, antibodies react against proteins in your thyroid gland, causing gradual destruction of the gland itself, and making it unable to produce the thyroid hormones your body needs.

Other Causes

Besides Hashimoto's disease, the 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 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. 

Certain Drugs

Certain medications can 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

Not enough iodine or too much can cause an underactive thyroid. 

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 may result in an underactive thyroid gland. 

Abnormal Deposition

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. 

Genetics

Common Risk Factors

Sources:

American Thyroid Association. (2018). Hypothyroidism (Underactive).

Braverman, L, Cooper D. Werner & Ingbar's The Thyroid, 10th Edition. WLL/Wolters Kluwer; 2012.

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.