The most common cause of hypothyroidism is Hashimoto’s thyroiditis. “Thyroiditis” is an inflammation of the thyroid gland. Hashimoto’s thyroiditis is an autoimmune disorder. With Hashimoto’s, your body produces antibodies that attack and destroy the thyroid gland. Thyroiditis may also be caused by a viral infection. Other causes of hypothyroidism include:

Radiation therapy to the neck area. Treating certain cancers, such aslymphoma, requires radiation to the neck. Radiation damages the cells in the thyroid. This makes it more difficult for the gland to produce hormone.

Radioactive iodine treatment. This treatment is commonly prescribed to people who have an overactive thyroid gland, a condition known as hyperthyroidism. However, radiation destroys the cells in the thyroid gland. This usually leads to hypothyroidism.

Use of certain medications. Certain medicines to treat heart problems, psychiatric conditions, and cancer can sometimes affect the production of thyroid hormone. These include amiodarone (Cordarone), lithium,interferon alpha, and interleukin-2.

Thyroid surgery . Surgery to remove the thyroid will lead to hypothyroidism. If only part of the thyroid is removed, the remaining gland may still be able to produce enough hormone for the body’s needs.

Too little iodine in the diet. The thyroid needs iodine to produce thyroid hormone. Your body doesn’t make iodine, so you need to get it through your diet. Iodized table salt is rich in iodine. Other food sources of iodine include shellfish, saltwater fish, eggs, dairy products, and seaweed. Iodine deficiency is rare in the U.S.

Pregnancy . The reason isn’t clear, but sometimes,inflammation of the thyroid occurs after pregnancy. This is called postpartum thyroiditis. Women with this condition usually have a severe increase in thyroid hormone levels followed by a sharp drop in thyroid hormone production. Most women with postpartum thyroiditis will regain their normal thyroid function.

Problems with the thyroid at birth. Some babies may be born with a thyroid gland that did not develop correctly or does not work properly. This type of hypothyroidism is called congenital hypothyroidism. Most hospitals in the U.S. screen babies at birth for this disease.

Pituitary gland damage or disorder. Rarely, a problem with the pituitary gland can interfere with the production of thyroid hormone. The pituitary gland makes a hormone, called thyroid-stimulating hormone (TSH), which tells your thyroid how much hormone it should make and release.

Disorder of the hypothalamus. An extremely rare form of hypothyroidism can occur if the hypothalamus in the brain does not produce enough of a hormone called TRH. TRH affects the release of TSH from the pituitary gland.

Primary hypothyroidism is caused by a problem with the thyroid gland itself.

Secondary hypothyroidism occurs when another problem interferes with the thyroid’s ability to produce hormones. For example, the pituitary gland and hypothalamus produce hormones that trigger the release of thyroid hormone. A problem with one of these glands can make your thyroid underactive.

Sometimes, an underactive thyroid that results from a problem with the hypothalamus is called tertiary hypothyroidism.

Who Is at Risk for Hypothyroidism?

Women, particularly older women, are more likely to develop hypothyroidism than men. You are also more likely to develop hypothyroidism if you have a close family member with an autoimmune disease. Other risk factors include:

• Race (being white or Asian)
• Age (growing older)
• Prematurely graying hair
• Autoimmune disorders such as type 1 diabetes, multiple sclerosis,rheumatoid arthritis, celiac disease, Addison’s disease, pernicious anemia, or vitiligo
• Bipolar disorder
• Down syndrome
• Turner syndrome

Symptoms of Hypothyroidism

Symptoms of hypothyroidism may be vague and can often mimic other conditions. They may include:

• Changes in the menstrual cycle
• Constipation
• Depression
• Dry hair and hair loss
• Dry skin
• Fatigue
• Greater sensitivity to cold
• Slow heart rate
• Swelling of the thyroid gland (goiter)
• Unexplained weight gain or difficulty losing weight
• Carpal tunnel syndrome

Babies with hypothyroidism may have no symptoms. If symptoms do occur, they can include:

• Cold hands and feet
• Constipation
• Extreme sleepiness
• Hoarse cry
• Little or no growth
• Low muscle tone (floppyinfant)
• Persistent jaundice (yellowing of the skin and whites of the eyes)
• Poor feeding habits
• Puffy face
• Stomach bloating
• Swollen tongue

Make an appointment with your health care provider if you or your baby has any of these symptoms. It is important to note that these symptoms can be due to other medical conditions.

How Is Hypothyroidism Diagnosed?

If you have symptoms of hypothyroidism, your doctor will order bloodtests to check hormone levels. These may include:

• Thyroid-stimulating hormone (TSH)
• T4 (thyroxine)

Lower-than-normal T4 levels usually mean you have hypothyroidism. However, some people may have increased TSH levels while having normal T4 levels. This is called subclinical (mild) hypothyroidism. It is believed to be an early stage of hypothyroidism.

If your test results or physical exam of the thyroid are abnormal, your doctor may order a thyroid ultrasound, or thyroid scan, to check for nodules or inflammation.

How Is Hypothyroidism Treated?

If you have hypothyroidism, your doctor will prescribe a synthetic (man-made) thyroid hormone T4. You take this pill every day. Certain other medications can interfere with how your body absorbs synthetic thyroid hormone. Make sure you doctor knows about all the medicines, herbs, and supplements you take, including over-the-counter products.

You will need regular blood tests to check your thyroid hormone levels. Your doctor may need to adjust your medication dose from time to time.

Complications of Hypothyroidism

• Heart problems
• Infertility
• Joint pain
• Obesity

Thyroid problems in a pregnant woman can affect the developing baby. During the first three months of pregnancy, the baby receives all thyroid hormone from its mother. If the mother has hypothyroidism, the baby does not get enough thyroid hormone. This can lead to problems with mental development.

Extremely low levels of thyroid hormone can cause a life-threatening condition called myxedema. Myxedema is the most severe form of hypothyroidism. A person with myxedema can lose consciousness or go into a coma. The condition can also cause the body temperature to drop very low, which can cause death.