Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone to meet the body's needs. Without enough thyroid hormone, many of the body's functions slow down. About 5 percent of the U.S. population has hypothyroidism. Women are much more likely than men to develop hypothyroidism.
What is the thyroid gland?
The thyroid is a 2-inch-long, butterfly-shaped gland weighing less than an ounce. It is located in the front of the neck below the larynx, or voice box, and comprises two lobes, one on either side of the windpipe. The thyroid is one of a group of glands that are part of the endocrine system. The endocrine glands produce, store, and release hormones into the bloodstream that travel through the body and direct the activity of the body's cells. Thyroid hormones regulate metabolism, which is the way the body uses energy, and affect nearly every organ in the body.
The thyroid gland makes two thyroid hormones, triiodothyronine (T3) and thyroxine (T4). Thyroid hormones affect metabolism, brain development, breathing, heart and nervous system functions, body temperature, muscle strength, skin dryness, menstrual cycles, weight, and cholesterol levels. A third hormone produced by specialized cells in the thyroid gland, calcitonin, affects calcium levels in the blood and the buildup of calcium in the bones. Calcitonin is not considered a thyroid hormone per se.
Thyroid hormone production is regulated by thyroid-stimulating hormone (TSH), which is made by the pituitary gland. Located in the brain, the pituitary gland is the "master gland" of the endocrine system.
What causes hypothyroidism?
Hypothyroidism has several causes, including
- Hashimoto's disease
- thyroiditis, or inflammation of the thyroid gland
- congenital hypothyroidism, or hypothyroidism that is present at birth
- surgical removal of part or all of the thyroid gland
- radiation treatment of the thyroid
- some medications
Less commonly, hypothyroidism is caused by too much or too little iodine in the diet or by abnormalities of the pituitary gland.
Hashimoto's disease, also called chronic lymphocytic thyroiditis, is the most common cause of hypothyroidism in the United States. Hashimoto's disease is an autoimmune disorder, which means the body's immune system, which normally protects the body by attacking foreign organisms, acts against its own healthy cells and tissues. In Hashimoto's disease, the immune system makes antibodies that attack cells in the thyroid and interfere with their ability to produce thyroid hormone.
Thyroiditis causes stored thyroid hormone to leak out of the inflamed thyroid gland. At first, the leakage raises hormone levels in the blood, leading to hyperthyroidism that lasts for a month or two. Most people then develop hypothyroidism before the thyroid is completely healed. Several types of thyroiditis can lead to hypothyroidism:
Subacute thyroiditis. This condition involves painful inflammation and enlargement of the thyroid. Doctors aren't sure what causes subacute thyroiditis, but it may be related to a viral or bacterial infection. The condition usually goes away on its own in a few months.
Postpartum thyroiditis. About 8 percent of women who have been pregnant develop postpartum thyroiditis within a few months of giving birth.2 In some women, the thyroid does not heal and their hypothyroidism is permanent. Postpartum thyroiditis is believed to be an autoimmune condition.
Silent thyroiditis. This type of thyroiditis is called "silent" because it is painless, as is postpartum thyroiditis, even though the thyroid may be enlarged. Silent thyroiditis is probably an autoimmune condition and sometimes develops into permanent hypothyroidism.
Some babies are born with a thyroid that is not fully developed or does not function properly. If untreated, congenital hypothyroidism can lead to mental retardation and growth failure. Most newborns in the United States are screened for hypothyroidism, and early treatment can prevent these complications.
Surgical Removal of the Thyroid
Part or all of the thyroid gland may be surgically removed as a treatment for
- hyperthyroidism, when the thyroid makes too much thyroid hormone
- a large goiter, which is an enlarged thyroid gland that may cause the neck to appear swollen and can interfere with normal breathing and swallowing
- thyroid nodules, which are lumps in the thyroid that can produce excess thyroid hormone
- thyroid cancer
When part of the thyroid is removed, the remaining part may produce normal amounts of thyroid hormone, but some people who have this surgery develop hypothyroidism. Removal of the entire thyroid always results in hypothyroidism.
Radiation Treatment of the Thyroid
Radioactive iodine, a common treatment for hyperthyroidism, gradually destroys the cells of the thyroid. Almost everyone who receives radioactive iodine treatment eventually develops hypothyroidism. People with Hodgkin's disease, other lymphomas, and head or neck cancers are treated with radiation, which can also damage the thyroid.
Some drugs can interfere with thyroid hormone production and lead to hypothyroidism. These drugs include
2Nicholson WK, Robinson KA, Smallridge RC, Ladenson PW, Powe NR. Prevalence of postpartum thyroid dysfunction: a quantitative review. Thyroid. 2006;16(6):573-582.
Source: National Endocrine and Metabolic Diseases Information Service