What Causes Hypothyroidism?


Erin Azuse, RN BSN

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Thyroid function examination

For being a relatively small part of your body, your thyroid controls a lot. This butterfly-shaped gland located at the front of your neck produces hormones that regulate how your cells use energy. When your thyroid does not make enough thyroid hormone, this is called hypothyroidism.

It is estimated that 4.6% of those over age twelve in the United States have hypothyroidism. Of these individuals, the majority are women. These numbers may lead you to wonder just what causes this disease. Let’s take a look at some of the reasons hypothyroidism develops.

Autoimmune Disorder / Hashimoto’s Disease

Hashimoto’s Disease is the most common cause of hypothyroidism. It’s a form of thyroiditis, or inflammation of the thyroid, that occurs when your immune system produces antibodies that attack the thyroid gland. This causes it to produce fewer hormones. It’s unknown why this autoimmune response occurs, though there is some speculation it may be triggered by a prior infection or due to a genetic predisposition.


Some patients with hyperthyroidism, a condition where too much thyroid hormone is produced, are treated with radioactive iodine. Radioactive iodine damages the thyroid gland, thereby decreasing its ability to create thyroid hormone. However, in some cases, too many thyroid cells are killed, leaving the patient with the opposite problem- hypothyroidism.

Additionally, patients who receive external radiation treatment for cancers of the head and neck may suffer damage to the thyroid gland.

Surgical Removal of Thyroid

Similarly, there are situations that warrant the removal of the thyroid gland. This may be recommended for:

  • Severe hyperthyroidism

  • Cysts, nodules or benign tumors on the thyroid

  • Thyroid cancer

  • Swelling of the thyroid (goiter) that causes problems with breathing or swallowing.

If the entire thyroid or a large portion of the thyroid gland is removed, the patient can become hypothyroid and will require lifetime thyroid hormone replacement therapy.  


Certain medications can lead to hypothyroidism.

  • Anti-thyroid medications- propylthiouracil (PTU) and methimalzole (Tapazole): Over-treatment of hyperthyroid with these medications can result in low levels of thyroid hormone.

  • Amiodorone: High levels of iodine in this medication used to treat abnormal heart rhythms can affect the thyroid.

  • Lithium: Used to treat bipolar disease, it also inhibits thyroid hormone secretion.

  • Interferon alpha and interleukin 2: These cancer-treating medications can cause an autoimmune response leading to hypothyroidism.

Iodine Deficiency or Excess

Our bodies do not naturally produce iodine, but it is necessary for the formation of thyroid hormone.  While this is generally not a problem in the United States due to the presence of iodine in bread and in our table salt, iodine deficiency is as issue in many other parts of the world and can result in hypothyroidism.

Conversely, taking in too much iodine can cause hypothyroidism as well.

Congenital Hypothyroidism

Some infants are born without a thyroid gland or have a defect within the gland, resulting in absent or decreased production of thyroid hormone (cretinism). If this is undetected, congenital hypothyroidism can lead to problems with growth and development. For this reason, newborn thyroid screening is now routinely performed in all 50 states.

Postpartum Thyroiditis

After delivery, some women develop an inflammation of the thyroid gland. Often a period of hyperthyroidism is first experienced, followed by a hypothyroid phase. For most, this will be a transient problem, but approximately 20% of women will remain hypothyroid.

It’s believed many people with thyroid disease don’t know they have it, and left untreated, complications can occur. If you’re experiencing signs of hypothyroidism, be sure to discuss it with your doctor. Luckily, with synthetic thyroid hormone treatment, thyroid levels often return to normal.

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Medical Reviewers: William C. Lloyd III, MD, FACS Last Review Date: Aug 16, 2016

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Medical References

  1. Hypothyroidism. MedlinePlus. https://medlineplus.gov/ency/article/000353.htm
  2. Hypothyroidism. Mayo Clinic. http://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/dxc-20155382
  3. Hypothyroidism and Hyperthyroidism. Cleveland Clinic. http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/endocrinology/hypothyroidism-and-h...
  4. Iodine Deficiency. American Thyroid Association. http://www.thyroid.org/iodine-deficiency/
  5. Thyroid Disease and Pregnancy. American Thyroid Association. http://www.thyroid.org/thyroid-disease-pregnancy/

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