An underactive thyroid, which doctors call hypothryoidism, occurs when the thyroid gland doesn't produce enough thyroid hormone to keep the body running normally. Mild forms are fairly common, particularly in older Americans and women. An underactive thyroid is easily treated, although most people having it must take medicine for a lifetime. Many illnesses are associated with underactive thyroid.
Hashimoto's Disease, Atrophic Thyroiditis
Hashimoto’s disease--a slowly progressing condition that attacks the thyroid gland--is the most common cause of underactive thyroid in the United States, according to MayoClinic.com. Atrophic thyroiditis, a rare immune system disorder, also causes hypothyroidism, according to the American Medical Association's Hypothryoidism.
Graves' Disease, Thyroid Cancer
People with other thyroid diseases, including Graves' disease--a condition caused by an overactive thyroid--and thyroid cancer may end up with an underactive thyroid after receiving treatment with radioactive iodine, reports Hypothyroidism. The iodine, taken orally, reaches the thyroid gland through the bloodstream and damages it, resulting in underactivity, according to MayoClinic.com.
Hodgkin's Disease
People with certain cancers develop underactive thyroid as a result of radiotherapy, reports Hypothryoidism. This includes those with head or neck cancer, lymphoma and Hodgkin’s disease.
Thyroiditis
Thyroiditis--inflammation of the thyroid gland--causes the thyroid gland to drop all of its stored hormone into the blood, resulting in brief overactivity, followed by hypothyroidism, reports Hypothyroidism. Thyroiditis can by caused by a viral infection or an autoimmune attack.
Congenital Defect
A birth defect in the thyroid causes hypothryoidism, says Hypothryoidism. Some babies are born with only part of a thyroid or none at all. Also, a baby may be born with the thyroid in a different location. Or, various thyroid chemicals or enzymes may not work correctly at birth.
Defects in Related Glands
Rarely, defects in other glands lead to hypothryoidism, MayoClinic.com says. This occurs if the pituitary gland fails to secrete enough thyroid-stimulating hormone, or TSH--the trigger for the thyroid to act--or if the hypothalamus secretes less thyrotropine-releasing hormone, which tells the pituitary gland how much TSH to make.
