The most common thyroid condition is hypothyroidism. When you are hypothyroid, your thyroid is not producing enough thyroid hormone — or you do not have a thyroid gland at all and are unable to produce any thyroid hormone. Hypothyroidism has a number of causes.
In autoimmune disease, your body manufactures antibodies to attack — and ultimately destroy — your own tissues, glands, organs, and cells. The most common autoimmune disease in the U.S. is Hashimoto's thyroiditis. With Hashimoto's thyroiditis, antibodies attack the thyroid gland and can impair or destroy your thyroid’s ability to produce sufficient thyroid hormone. Hashimoto’s thyroiditis is also the most common cause of hypothyroidism in the U.S.
Surgery to remove all or part of your thyroid gland — known as thyroidectomy — usually results in permanent, lifelong hypothyroidism. Thyroidectomy is used to treat thyroid cancer, large goiter, thyroid nodules that impair swallowing or breathing, and, less commonly, hyperthyroidism.
Radioactive iodine (RAI) treatment
RAI ablation is a treatment used for thyroid cancer patients after surgery to destroy any remnant thyroid tissue. For Graves’ disease patients, RAI is given to slow down the gland's ability to produce thyroid hormone, reversing hyperthyroidism. In most cases, after RAI treatment, patients will have lifelong hypothyroidism.
In some cases, babies are born without a thyroid gland, or their gland is malformed or malfunctioning and is unable to produce thyroid hormone. This condition is known as congenital hypothyroidism. Testing for congenital hypothyroidism is one of the mandated "heel-stick" tests performed on newborns in the U.S.
Iodine deficiency or excess
A deficiency of iodine, which is ingested from food, supplements, and iodized salt, is a cause of hypothyroidism. Hypothyroidism due to iodine deficiency is especially common in some areas of the world without salt iodization programs. Iodine deficiency and hypothyroidism in pregnant women pose a serious risk to their children, who are sometimes born with a condition called cretinism. Iodine deficiency is considered the leading preventable cause of mental retardation in the world.
Excessive exposure to iodine can also cause hypothyroidism. Overexposure to iodine can be due to overconsumption of iodine-rich foods, use of iodine supplements or supplements that contain iodine (including bladderwrack, kelp, and some thyroid support formulas), external iodine (i.e., povidone/Betadine solution), or intravenous iodine contrast agents used for diagnostic medical tests.
A number of prescription drugs can cause hypothyroidism. These include:
- Antithyroid drugs used to treat Graves’ disease and hyperthyroidism, including propylthiouracil (PTU), methimazole, and carbimazole
- Other drugs used to treat an overactive thyroid, including RAI and potassium iodide
- Lithium, used to treat bipolar disorder
- Amiodarone, used to treat heart arrhythmias
- Interferon alpha, used to treat hepatitis C and some cancers
- Interleukin-2, used to boost the immune system for cancer treatment
- Thalidomide, used to treat cancer
- Bexarotene, used to treat lymphoma
- Ipilimumab, used to treat melanoma
- Sunitinib and sorafenib, used to treat cancer
A number of supplements can cause hypothyroidism, including:
- Thyroid support formulas that are high in iodine
- Supplements that are high in iodine, such as kelp or bladderwrack
- Iodine supplements, such as Iodoral or Lugol’s solution
Goitrogens are chemicals found naturally in foods that can promote thyroid enlargement (goiter), and trigger hypothyroidism. The most goitrogenic foods include the cruciferous vegetables in the brassica family, such as kale, spinach, cabbage, and Brussels sprouts. For the cruciferous vegetables, the greatest risk is in consuming large quantities in raw form. Heat has been shown to significantly reduce the goitrogenic properties, making these foods safe to eat in moderate quantities after cooking or steaming.
Soy is also a goitrogen. In the case of soy, cooking and steaming do not reduce the goitrogenic potential. In addition, soy also has the ability to block the body’s ability to absorb thyroid hormone. Consumption of high levels of soy, taking high-dose soy supplements, or an entirely soy-based diet (such as an infant on soy-based formula) are all associated with an increased risk of hypothyroidism.
Chronic overexposure to certain chemicals is known to cause hypothyroidism. These chemicals include:
- Fluoride, which before its use as an additive to water supplies was used as a drug to treat hyperthyroidism
- Perchlorate, a chemical found in some water and food supplies as a result of contamination from rocket fuel production and the use of certain fertilizers
- Chemicals found in non-stick pans, fire retardant fabrics, and sprays, as well as certain pesticides that act as environmental estrogens
In some cases, a defect in or damage to the pituitary gland can cause hypothyroidism. This is known as central hypothyroidism. In central hypothyroidism, the pituitary gland is not able to produce any or enough thyroid stimulating hormone (TSH). This means that the thyroid gland is not stimulated to produce thyroid hormone, resulting in hypothyroidism. Two of the more common causes of pituitary dysfunction are tumors in the pituitary gland and a condition known as Sheehan's Syndrome.
Serious trauma to your neck, such as breaking your neck or a whiplash injury, has been associated with the onset of hypothyroidism. This is known as traumatic hypothyroidism.
The thyroid is especially sensitive to radiation. Radiation treatments to the head and neck — for example, radiation to treat Hodgkin's lymphoma — are associated with an increased risk of hypothyroidism. A history of extensive head, neck, or dental x-rays is also associated with an increased risk of hypothyroidism. In addition, exposure — especially in utero or as a child — to radioactive fallout (such as after the Chernobyl and Fukushima nuclear accidents) is linked to an increased risk of hypothyroidism.
In some cases, your thyroid gland can become underactive, but there is no identifiable cause, trigger, or disease your practitioner can identify. This is known as idiopathic hypothyroidism.
See more helpful articles:
The Risks for and Symptoms of Hypothyroidism
You Might Be Hypothyroid If...
How Hypothyroidism is Diagnosed