Low thyroid function, or hypothyroidism, is when the thyroid gland no longer produces enough thyroid hormone to maintain the normal functions of metabolism. A deficiency of thyroid hormone can have many different causes, but it is usually an autoimmune disorder. Hypothyroidism is common, especially in women over 65, according to Merck Manuals Medical Library.
Autoimmunity
Several types of low thyroid function can be caused by the body undergoing an autoimmune process, wherein the immune system mistakenly attacks its own thyroid tissue.
Hashimoto's thyroiditis is an autoimmune condition in which the body produces antibodies against the thyroid gland, causing its destruction. Hashimoto's disease is the most common cause of hypothydroidism. Hashimoto's almost always causes the thyroid to develop a goiter, a fibrous enlargement of the thyroid gland.
Atrophic thyroiditis is an autoimmune thyroid disorder similar to Hashimoto's thyroiditis, but there is usually no goiter present.
Riedel's thyroiditis is a rare autoimmune condition wherein normal thyroid tissue is taken over by scar tissue, creating a hard mass that can resemble thyroid cancer.
An autoimmune reaction during pregnancy can result in low thyroid function. The University of Maryland Medical Center states that as many as 10 percent of women develop hypothyroidism 4 to12 months after delivery.
Viruses
Subacute thyroiditis is a rare and painful inflammation of the thyroid gland that can make people feel ill for between 6 to 8 weeks. The thyroid first becomes overactive, then under active, and then usually returns to normal function. MedlinePlus describes an upper respiratory viral infection as the probable cause.
Treatment of Hyperthyroidism
Sometimes, people who have been treated with radioactive iodine for an overactive thyroid gland go on to develop hypothyroidism.
Iodine Disorder
Iodine is needed for the body to produce thyroid hormone. Getting either too much or too little iodine in the diet can cause low thyroid function.
Drugs
Certain drugs can cause low thyroid function. Lithium, alpha interferon and high doses of an iodine containing drug called amiodarone can all reduce thyroid function.
Radiation
People who have undergone radiation treatment of the head and neck are at greatly increased risk of developing hypothyroidism in the months and years following treatment, reports Merck Manuals Medical Library.
Congenital Hypothyroidism
Hypothyroidism may be present at birth due to an enzyme abnormality, or because of an underdeveloped thyroid gland.
Damage to the Pituitary Gland
Disease of the pituitary gland is a rare cause of low thyroid function.
