Causes
Thyroid diseases sometimes result from inappropriate TSH levels, or may be caused by problems in the thyroid gland itself.
The most common cause of hypothyroidism in the United States is Hashimoto's thyroiditis, an autoimmune condition where the body makes antibodies that destroy parts of the thyroid gland. Surgical removal and certain medications (e.g., amiodarone, lithium) can also cause hypothyroidism.
Other causes of hypothyroidism include pituitary problems, hypothalamus problems, and iodine deficiency (rare in North America, but affects nearly 2 billion people worldwide, according to the 2007 World Health Organization [WHO] report). Some babies are born with hypothyroidism - this is called congenital hypothyroidism.
There are different causes of hyperthyroidism. Graves' disease is the most common cause of hyperthyroidism. This condition occurs when the immune system produces an antibody that stimulates the entire thyroid gland; this leads to overactivity and higher levels of thyroid hormones.
Another form of hyperthyroidism is called toxic nodular goiter or toxic thyroid adenoma. Adenomas, abnormal nodules of tissue in the thyroid, constantly produce thyroid hormones even when they are not needed.
Secondary hyperthyroidism is caused when the pituitary gland makes too much TSH, leading to constant stimulation of the thyroid gland. A pituitary tumor may cause TSH levels to rise. More rarely, the pituitary gland becomes insensitive to thyroid hormones, no longer responding to high levels.
Another possible cause of hyperthyroidism is a condition called thyroiditis. This condition occurs when the thyroid gland becomes inflamed. Depending on the type of thyroiditis, this may lead to transient hyperthyroidism that might be followed by hypothyroidism.
There are four types of thyroid cancers: papillary, follicular, anaplastic, and medullary cancer. These are associated with radiation treatment to the head, neck, or chest. Radiation treatment for benign (non-cancerous) conditions is no longer carried out in these areas, but was more common in the past. In other cases, a genetic mutation might by associated with thyroid cancer, either alone or in conjunction with other types of cancers (e.g., multiple endocrine neoplasia, BRAF gene mutations). Less commonly, other cancers might metastasize to the thyroid (e.g., lymphoma, breast cancer).