Causes of Hypothyroidism

Autoimmune disease. The immune system that protects the body against infection may mistake thyroid cells and their enzymes for foreign aggressors and attack them, leaving insufficient cells to produce the necessary amount of thyroid hormone. It is a disease that can begin acutely or develop slowly over many years, and it is most typically seen in women. The most common forms are Hashimoto's thyroiditis and atrophic thyroiditis.  

Partial or total removal of the thyroid by surgery. Some people with thyroid nodules, thyroid cancer or Graves' disease need surgery to remove part or all of their thyroid gland. If the entire gland is removed, the person will develop hypothyroidism. If part of the thyroid is left intact, it may produce enough thyroid hormone to keep blood levels within the normal range. 

Radioactive iodine treatment. In people treated with radioactive iodine for other thyroid diseases (Graves' disease, nodular goitre or thyroid cancer), as well as those who have undergone therapeutic irradiation to the neck or head (cancer, lymphoma, etc.), some or all of their thyroid function may be lost.

Congenital hypothyroidism (hypothyroidism in newborns). Some children are born without a thyroid or with a partially formed or abnormally located (ectopic) gland, or with thyroid cells or enzymes that do not function normally, which can lead to hypothyroidism.

Thyroiditis. This is an inflammation of the thyroid gland caused by an autoimmune attack or a viral infection. In its first phase, this inflammation leads to the release of thyroid hormone from within the gland, causing hyperthyroidism and, subsequently, a decrease in thyroid function, leading to hypothyroidism. This can be permanent (chronic thyroiditis) or temporary (subacute thyroiditis, postpartum thyroiditis).  

Medications (amiodarone, lithium, interferon, interleukin, immunotherapy). These cause hypothyroidism more frequently in patients with a predisposition to developing autoimmune thyroid disease. 

Iodine excess or deficiency. Thyroid hormones contain a large amount of iodine. This requires a balanced iodine intake, as excessive iodine, as well as iodine deficiency, can cause or aggravate hypothyroidism. 

Pituitary disease. The pituitary gland tells the thyroid gland how much thyroid hormone to produce. If the pituitary gland is not functioning and does not stimulate the thyroid, the thyroid gland stops producing enough hormone. 

Rare infiltrative thyroid diseases. Some diseases such as amyloidosis, sarcoidosis, and haemochromatosis cause an abnormal substance to build up in the thyroid (amyloid, granulomas, or iron, respectively). This prevents it from functioning normally.