Causes of Hypothyroidism

Hashimoto’s thyroiditis is the most common cause

The most common cause of hypothyroidism is Hashimoto's thyroiditis (or autoimmune hypothyroidism), a form of thyroid inflammation caused by your own immune system. But this isn't the sole cause of hypothyroidism—there may be a variety of other reasons why you have developed the disorder. This article gives an explanation of each of the possible causes of hypothyroidism.

Hashimoto's Thyroiditis
Hashimoto's thyroiditis is a common disorder. It affects as many as 10 million people in the US alone, and approximately 10% of women over age 30 have Hashimoto's thyroiditis (the disease affects women ten times more than men).

Hashimoto's thyroiditis is characterized by thyroid inflammation due to attacks from the body's own immune system on the thyroid gland. This causes the thyroid's tissue to gradually decay, which directly impacts the thyroid's ability to produce hormones. This will eventually result in hypothyroidism when the thyroid is unable to make the amount of thyroid hormone that your body needs.

When the body's immune system attacks its own healthy tissue, it's known as an autoimmune disorder. If you have an autoimmune disorder, such as rheumatoid arthritis, type 1 diabetes, or Addison's disease, and have symptoms of hypothyroidism, you should talk to your doctor about also getting checked for Hashimoto's thyroiditis.

Problems with the Thyroid Itself
If your thyroid has been surgically removed, you'll develop hypothyroidism. Fortunately, your doctor will start you on a course of thyroid hormone replacement medications before you even begin to experience hypothyroid symptoms.

Hypothyroidism may also be caused by congenital thyroid agenesis (being born without a thyroid), and the destruction of the thyroid by radioactive iodine (radioactive iodine therapy is a common treatment for thyroid cancer patients and patients with hyperthyroidism).

Medications
Some medications may cause hypothyroidism, including:

  • Amiodarone: Used to treat heart rhythm conditions, this drug has a high iodine content, which can inhibit the synthesis and release of thyroid hormone and also conversion of peripheral conversion of T4 to T3, causing hypothyroidism in 5-20% of the patients. Iodine is critical to thyroid health—but too much or too little can cause it to stop working correctly.
  • Anti-thyroid medications: These medications treat overactive thyroid glands. Doctor supervision is highly important, as taking these drugs at too high a dose can result in hypothyroidism.
  • Interferon-alpha: Patients with certain malignant tumors, as well as those with hepatitis B and C, use this medication. A small amount of patients develop a thyroid disorder, such as hypothyroidism, as a result.
  • Interleukin-2 (IL-2): This medication may be prescribed to patients with some metastatic cancers and leukemia. About 2% of patients on this drug develop a thyroid disorder.
  • Lithium: This drug treats depression and bipolar disorder. But because lithium slows the production and release of thyroid hormones, a significant amount of patients—20-30%, in fact—develop hypothyroidism.

Abnormal Growths in the Thyroid
It's possible that abnormal growths can "invade" the thyroid and take the place of healthy tissue. There are a few disorders that cause this, and one example is an autoimmune disorder called sarcoidosis. In sarcoidosis, inflamed tissue forms throughout the body. This inflammation can replace healthy thyroid tissue, which will inhibit thyroid hormone production. This eventually results in hypothyroidism.

Problems in the Pituitary
A rare cause of hypothyroidism may have little do to with the actual thyroid gland and more to do with the gland that actually controls it—the pituitary. A problem in the pituitary gland (such as a tumor) may impact its ability to produce thyroid stimulating hormone (TSH). Without TSH, the thyroid won't produce and secrete hormones because it doesn't have the "signal" from the pituitary.

Genetic Defects
In rare cases, the genes that regulate thyroid hormones may be damaged. This directly affects thyroid hormone production, thus possibly causing hypothyroidism. Most genetic causes of hypothyroidism are apparent at birth or early infancy.

Though Hashimoto's thyroiditis is the most common cause of hypothyroidism, you may have developed the disorder a variety of other ways. What's most important is to start discussing the next steps to take with your doctor. He or she will help you get started on a treatment plan that will effectively manage your hypothyroid symptoms.