Hypothyroidism is a problem in which the thyroid is underactive and is very common, especially among women. At least 10% of women in the United States have signs of hypothyroidism by the age of 50. The risk of hypothyroidism increases with age and at age 60, 17% of women and 8% of men have signs of hypothyroidism. The most common cause of hypothyroidism is an autoimmune problem (i.e. the body attacks specific parts of itself) called Hashimoto's thyroiditis or lymphocytic thyroiditis. In this disease, the body makes antithyroid antibodies that attack and destroy the thyroid. Another common cause of hypothyroidism is thyroid surgery. Removal of the whole thyroid gland (i.e. total thyroidectomy) will definitely cause hypothyroidism and up to 30 to 50% of patients having half of the thyroid removed (i.e. thyroid lobectomy) will develop hypothyroidism. By removing the thyroid, the patient can no longer make thyroid hormone.
Unless the patient is given thyroid hormone in pill form, they are by definition hypothyroid. Similarly, radioactive iodine ablation (RAI ablation) and anti-thyroid medications are designed to stop the thyroid from making thyroid hormone and will cause hypothyroidism. Other medications such as lithium (used for psychiatric diseases such as bipolar disorder) and amiodarone (used for certain irregular heart rhythms) can cause hypothyroidism.