What causes hypothyroidism?
Hypothyroidism is a very common condition. Approximately 3% to 4% of the U.S. population has some form of hypothyroidism. This type of thyroid disorder is more common in women than in men, and its incidence increases with age. Examples of common causes of hypothyroidism in adults include Hashimoto's thyroiditis, an autoimmune form of overactive thyroid, lymphocytic thyroiditis, which may occur after hyperthyroidism (underactive thyroid), thyroid destruction from radioactive iodine or surgery, pituitary or hypothalamic disease, medications, and severe iodine deficiency.
The most common cause of hypothyroidism in the United States is an inherited condition called Hashimoto's thyroiditis. This condition is named after Dr. Hakaru Hashimoto who first described it in 1912. In this condition, the thyroid gland is usually enlarged (goiter) and has a decreased ability to make thyroid hormones. Hashimoto's is an autoimmune disease in which the body's immune system inappropriately attacks the thyroid tissue. In part, this condition is believed to have a genetic basis. This means that the tendency toward developing Hashimoto's thyroiditis can run in families. Hashimoto's is 5 to 10 times more common in women than in men.
Severe iodine deficiency
In areas of the world where there is an iodine deficiency in the diet, severe hypothyroidism occurs in about 5% to 15% of the population. Examples of these areas include Zaire, Ecuador, India, and Chile. Severe iodine deficiency occurs in remote mountain areas such as the Andes and the Himalayas. Since the addition of iodine to table salt and bread, iodine deficiency is rare in the United States.
Lymphocytic thyroiditis following hyperthyroidism
Thyroiditis refers to inflammation of the thyroid gland. Lymphocytic thyroiditis is a condition in which the inflammation is caused by a particular type of white blood cell known as a lymphocyte. Lymphocytic thyroiditis is particularly common after pregnancy and can affect up to 8% of women after they deliver their baby. In this type of thyroid disorder there usually is a hyperthyroid phase (in which excessive amounts of thyroid hormone leak out of the inflamed gland), which is followed by a hypothyroid phase that can last for up to six months. In the majority of women with lymphocytic thyroiditis, the thyroid eventually returns to its normal function, but there is a possibility that the thyroid will remain underactive.
Thyroid destruction secondary to radioactive iodine or surgery
People who have been treated for hyperthyroidism (underactive thyroid) like Graves' disease, and received radioactive iodine may be left with little or no functioning thyroid tissue after treatment. The likelihood of this depends on several factors including the dose of iodine given, along with the size and the activity of the thyroid gland. If there is no significant activity of the thyroid gland six months after the radioactive iodine treatment it usually means that the thyroid gland is no longer functioning adequately. The result is hypothyroidism. Similarly, removal of the thyroid gland during surgery causes hypothyroidism.
Pituitary gland or hypothalamic disease
If for some reason the pituitary gland or the hypothalamus is unable to signal the thyroid and instruct it to produce thyroid hormones, it may cause decreased T4 and T3 blood levels, even if the thyroid gland itself is normal. If pituitary disease causes this defect, the condition is called "secondary hypothyroidism." If the defect is due to hypothalamic disease, it is called "tertiary hypothyroidism."
Pituitary gland injury
A pituitary injury may result after brain surgery or the blood supply to the brain has decreased. When the pituitary gland is injured, hypothyroidism results in low TSH blood levels because the thyroid gland is no longer stimulated by the pituitary TSH. Usually, hypothyroidism from pituitary gland injury occurs in together with other hormone deficiencies, since the pituitary regulates other processes such as growth, reproduction, and adrenal function.
Pituitary gland injury from medications
Medications that are used to treat an overactive thyroid (hyperthyroidism) may cause hypothyroidism. These drugs include methimazole (Tapazole) and propylthiouracil (PTU). The psychiatric medication, lithium (Eskalith, Lithobid), is also known to alter thyroid function and cause hypothyroidism. Interestingly, drugs containing a large amount of iodine such as amiodarone (Cordarone), potassium iodide (SSKI, Pima), and Lugol's solution can cause changes in thyroid function, which may result in low blood levels of thyroid hormone.