Primary hypothyroidism occurs when there is a problem with the thyroid gland itself. The most common cause of adult hypothyroidism is Hashimoto's thyroiditis. It's caused by an autoimmune process where the body produces antibodies that attack and gradually destroy the thyroid gland.
Women are eight times more likely than men to develop Hashimoto's thyroiditis, especially as they age. It can also run in families or be associated with syndromes of genetic abnormalities such as Turner's syndrome, Klinefelter's syndrome, and Down syndrome.
Hypothyroidism can also be caused by treatments for hyperthyroidism. To treat hyperthyroidism, the thyroid gland may be rendered inactive with medications or radioactive iodine treatment, or it may be surgically removed. The result may be a lack of thyroid hormones, causing hypothyroidism.
The thyroid gland requires iodine to function properly. A chronic lack of iodine means that less thyroid hormone can be produced and this causes the thyroid to enlarge. Since salt manufacturers now add iodine to salt, this form of hypothyroidism is extremely rare in North America. However, it is still the major cause of hypothyroidism in underdeveloped countries, where iodine is often lacking in the diet.
Some rare inherited disorders cause enzyme abnormalities in the thyroid gland that don't allow it to make the hormones. Secondary hypothyroidism (when there is a problem with the pituitary or hypothalamus, not the thyroid) may occur if the pituitary gland in the brain isn't working properly, and not producing a hormone (called thyroid-stimulating hormone (TSH)) needed to stimulate the thyroid gland. These disorders are quite rare and not a major cause of hypothyroidism.
Some medications can cause a person to develop hypothyroidism by interfering with the production of the thyroid hormone. These include amiodarone* (a heart medication), lithium (a bipolar disorder medication), and interferon alpha (a cancer medication).