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Causes of Hypothyroidism

Hypothyroidism is a very common condition. It is estimated that 3 to 5% of the population has some form of hypothyroidism. The condition is more common in women than in men, and its incidence increases with age.

Some of the common causes of hypothyroidism in adults are:

Hashimoto's Thyroiditis

The most common cause of thyroid gland failure is called autoimmune thyroiditis, also called Hashimoto's thyroiditis, a form of thyroid inflammation caused by the patient's own immune system. This condition is named after Dr. Hakaru Hashimoto who first described it in 1912. In part, this condition is believed to have a genetic basis. Hashimoto's is 5 to 10 times more common in women than in men.

Lymphocytic Thyroiditis

Thyroiditis refers to inflammation of the thyroid gland. When a particular type of white blood cell known as a lymphocyte causes the inflammation, the condition is referred to as lymphocytic thyroiditis. This condition is particularly common after pregnancy and can actually affect up to 8% of women after they deliver. In these cases, there is usually a phase in which excessive amounts of thyroid hormone leak out of the inflamed gland also called hyperthyroid, which is followed by a hypothyroid phase that can last for up to 6 months. The majority of affected women eventually return to a state of normal thyroid function, although there is a possibility of remaining hypothyroid.

Thyroid destruction

Thyroid destruction secondary to radioactive iodine or surgery, as treatment for hyperthyroidism can also cause hypothyroidism. Patients who have been treated for a hyperthyroid condition (such as Graves' disease) and received radioactive iodine may be left with little or no functioning thyroid tissue after treatment. If there is no significant activity of the thyroid gland six months after the radioactive iodine treatment, it is usually assumed that the thyroid will no longer function adequately. The result is hypothyroidism. Similarly, removal of the thyroid gland during surgery will be followed by hypothyroidism.

Pituitary or Hypothalamic disease

Pituitary or Hypothalamic disease can also lead to condition of hypothyroidism. If for some reason the pituitary gland or the hypothalamus is unable to signal the thyroid and instruct it to produce thyroid hormones, a decreased level of circulating T4 and T3 may result, even if the thyroid gland itself is normal. If this defect is caused by pituitary disease, the condition is called "secondary hypothyroidism." If the defect is due to hypothalamic disease, it is called "tertiary hypothyroidism."

Pituitary Injury

A pituitary injury may result after brain surgery or if there has been a decrease of blood supply to the area. In these cases of pituitary injury, the TSH that is produced by the pituitary gland is deficient and blood levels of TSH are low. Because the thyroid gland is no longer stimulated by the pituitary TSH, hypothyroidism results.


Medications that are used to treat an over-active thyroid (hyperthyroidism) may actually cause hypothyroidism. These drugs include Methimazole (Tapezole) and Propylthiouracil (PTU). The psychiatric medication, lithium, is also known to alter thyroid function and cause hypothyroidism.

Severe Iodine Deficiency

Severe Iodine Deficiency is another major cause of hypothyroidism. In areas of the world where there is an iodine deficiency in the diet, severe hypothyroidism can be seen in 5 to 15% of the population. Examples of these areas include Zaire, Ecuador, India, and Chile.

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