What are the causes of an underactive thyroid condition?
In the United States, the most common cause is an autoimmune disease (a disease in which the immune system sees one’s own thyroid as foreign and attacks it) called Hashimoto’s disease. Other causes are surgical, drug, or radiation treatment of thyroid problems that have a side effect of lowering thyroid hormone production. Other rare causes include pituitary and other brain diseases that effect the brain’s ability to regulate thyroid hormone production. In poorer parts of the world, where good nutrition is difficult, a shortage of iodine in the diet can also cause low thyroid levels since iodine is necessary for thyroid hormone production. In the USA, iodine is available in most table salts to be sure that no shortages occur (“iodized” salt).
