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.
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 of developing hypothyroidism.
Thyroiditis
Thyroiditis describes thyroid gland inflammation and is a general term for a variety of thyroid conditions.
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:
LithiumAmiodaroneThionamides (antithyroid drugs)Interferon-alphaInterleukin-2Certain 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 hormones. 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.
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 help 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)
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.
