Hypothyroidism occurs when the thyroid gland releases too little thyroid hormone.. Numerous disorders and conditions may cause hypothyroidism. These include:

• Thyroiditis. Inflammation of the thyroid gland destroys thyroid cells and allows excess hormones to leak into the bloodstream. At first, the excess hormones may produce hyperthyroidism, but as the thyroid cells are destroyed by inflammation, they are unable to produce hormones and hypothyroidism sets in. In some cases, thyroiditis is caused by an autoimmune disease, which occurs when antibodies (protective proteins) mistakenly attack the gland, damaging thyroid cells. Examples include Hashimoto's thyroiditis and atrophic thyroiditis, the disorders most commonly associated with hypothyroidism. People who have an autoimmune disease, such as type 1 diabetes, are at increased risk of developing other autoimmune disorders. Thyroiditis also can result from pregnancy (see below) or a viral infection.
  
  
• Hyperthyroidism treatments. Patients who produce too much thyroid hormone (hyperthyroidism) may be treated with medications designed to reduce such hormone production. In some cases, production may be curbed too much, leading to hypothyroidism. After treatment of an overactive thyroid with radioactive iodine, enough thyroid tissue may be lost as to result in hypothyroidism. Likewise, patients who require surgical removal of part or all of the thyroid often require thyroid hormone supplements post-operatively to prevent hypothyroidism.
  
  
• Iodine deficiency. Lack of dietary iodine, a trace mineral, can lead to hypothyroidism, although this rarely occurs in the United States. Iodine is found primarily in iodized salt, seafood, seaweed and plants grown in iodine-rich soil. Excessive levels of iodine paradoxically can also lead to hypothyroidism.
  
  
• Medications. Various medications can contribute to hypothyroidism, including amiodarone (antiarrhythmic heart drug), lithium (bipolar disorder drug), interferon alpha (cancer drug), interleukin-2 (biological response modifier). Growth hormone, a treatment for some causes of dwarfism in children, can cause temporary hypothyroidism.
  
  
• Pituitary disorder. In some cases, a patient's pituitary gland may fail to produce adequate levels of thyroid-stimulating hormone (TSH). This rare condition can lead to hypothyroidism and is usually the result of a benign tumor. Radiation therapy and surgery also can cause this type of disorder.
  
  
• Pregnancy. Women who are pregnant may produce antibodies to their own thyroid gland, leading to hypothyroidism that increases the risk of miscarriage, premature delivery and preeclampsia. In addition, some women will have a bout of hyperthyroidism followed by hypothyroidism shortly after giving birth. This is known as postpartum thyroiditis and is usually temporary.
  
  
• Radiation therapy. Radiation treatments used to treat cancers of the head and neck can damage the thyroid gland, leading to hypothyroidism.
  
  
• Genetic disorders. Turner syndrome, which affects only females, typically involves short stature and dysfunctional ovaries and can lead to hypothyroidism or other thyroid problems, diabetes, cardiovascular conditions, high blood pressure and osteoporosis. Children with Down syndrome, a chromosomal disorder that causes mental retardation, also have a greater risk of hypothyroidism or other thyroid problems.

• Other conditions. Some illnesses can leave substances in the thyroid that impair the functioning of this gland. These illnesses include infections, amyloidosis (abnormal buildup of proteins in organs, sometimes resulting from kidney dialysis), sarcoidosis (disease marked by inflammatory lumps in the lungs and other organs) and hemochromatosis. Head injury or other trauma can also impair thyroid function.

Although hypothyroidism can affect anyone, women (especially those over 50) are at greater risk than men. The risk increases with age. However, in some cases, children are born with no thyroid gland, a defective thyroid or a thyroid in the wrong place (ectopic thyroid). All of these conditions can lead to hypothyroidism.

Other risk factors for hypothyroidism include:

• Family history of autoimmune disease.
  
  
• Individual history of treatment with radioactive iodine, antithyroid medications or radiation therapy to the neck and upper chest.
  
  
• Individual history of surgery on the thyroid.
  
  
• Obesity.
  
  
• Exposure to chemicals. Women exposed to perchlorate, a common industrial chemical that has been found in U.S. drinking water, produce and milk, may be at risk for reduced thyroid function, the Centers for Disease Control and Prevention (CDC) reported in 2006.

Most patients who are diagnosed with hypothyroidism will have the condition for life, although underactive thyroid associated with pregnancy or viral illness is more likely to disappear over time.