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Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.

See also:

• Chronic thryoiditis (Hashimoto's disease)
• Subacute thyroiditis
• Silent thyroiditis
• Neonatal hypothyroidism

The thyroid gland is located in the front of the neck just below the voice box (larynx). It releases hormones that control metabolism.

The most common cause of hypothyroidism is inflammation of the thyroid gland, which damages the gland's cells. Autoimmune or Hashimoto's thyroiditis, in which the immune system attacks the thyroid gland, is the most common example of this. Some women develop hypothyroidism after pregancy (often referred to as "postpartum throiditis").

Other common causes of hypothyroidism include:

• Congenital (birth) defects
• Radiation treatments to the neck to treat different cancers, which may also damage the thyroid gland
• Radioactive iodine used to treat an overactive thyroid (hyperthyroidism)
• Surgical removal of part or all of the thyroid gland, done to treat other thyroid problems
• Viral thyroiditis, which may case hyperthyroidism and is often followed by temporary or permanent hypothyroidism

Certain drugs can cause hyperthyroidism, including:

• Amiodarone
• Drugs used for hyperthyroidism (overactive thyroid), such as propylthiouracil (PTU) and methimazole
• Lithium
• Radiation to the brain
• Sheehan syndrome, a condition that may occur in a woman who bleeds severely during pregnancy or childbirth and causes destruction of the pituitary gland

Risk factors include:

• Age over 50 years
• Being female

Early symptoms:

• Being more sensitive to cold
• Constipation
• Depression
• Fatigue or feeling slowed down
• Heavier menstrual periods
• Joint or muscle pain
• Paleness or dry skin
• Thin, brittle hair or fingernails
• Weakness
• Weight gain (unintentional)

Late symptoms, if left untreated:

• Decreased taste and smell
• Hoarseness
• Puffy face, hands, and feet
• Slow speech
• Thickening of the skin
• Thinning of eyebrows

A physical examination may reveal a smaller-than-normal thyroid gland, although sometimes the gland is normal size or even enlarged (goiter). The examination may also reveal:

• Brittle nails
• Coarse facial features
• Pale or dry skin, which may be cool to the touch
• Swelling of the arms and legs
• Thin and brittle hair

A chest x-ray may show an enlarged heart.

Laboratory tests to determine thyroid function include:

• Serum TSH
• T4 test

Lab tests may also reveal:

• Anemia on a complete blood count (CBC)
• Increased cholesterol levels
• Increased liver enzymes
• Increased prolactin
• Low sodium

The purpose of treatment is to replace the thyroid hormone that is lacking. Levothyroxine is the most commonly used medication. Doctors will prescribe the lowest dose that effectively relieves symptoms and brings the TSH level to a normal range. If you have heart disease or you are older, your doctor may start with a very small dose.

Lifelong therapy is required unless you have a condition called transient viral thyroiditis.

You must continue taking your medication even when your symptoms go away. When starting your medication, your doctor may check your hormone levels every 2 - 3 months. After that, your thyroid hormone levels should be monitored at least every year.

Important things to remember when you are taking thyroid hormone are:

• Do NOT stop taking the medication when you feel better. Continue taking the medication exactly as directed by your doctor.
• If you change brands of thyroid medicine, let your doctor know. Your levels may need to be checked.
• Some dietary changes can change the way your body absorbs the thryoid medicine. Talk with your doctor if you are eating a lot of soy products or a high-fiber diet.
• Thryoid medicine works best on an empty stomach and when taken 1 hour before any other medications. Do NOT take thyroid hormone with calcium, iron, multivitamins, alumin hydroxide antacids, colestipol, or other medicines that bind bile acids, or fiber supplements.

After you start taking replacement therapy, tell your doctor if you have any symptoms of increased thyroid activity (hyperthyroidism) such as:

• Rapid weight loss
• Restlessness or shakiness
• Sweating

Myxedema coma is a medical emergency that occurs when the body's level of thyroid hormones becomes extremely low. It is treated with intravenous thyroid hormone replacement and steroid medications. Some patients may need supportive therapy (oxygen, breathing assistance, fluid replacement) and intensive-care nursing.

In most cases, thyroid levels return to with proper treatment. However, thyroid hormone replacement must be taken for the rest of your life.

Myxedema coma can result in death.

Myxedema coma, the most severe form of hypothyroidism, is rare. It may be caused by an infection, illness, exposure to cold, or certain medications in people with untreated hypothyroidism.

Symptoms and signs of myxedema coma include:

• Below normal temperature
• Decreased breathing
• Low blood pressure
• Low blood sugar
• Unresponsiveness

Other complications are:

• Heart disease
• Increased risk of infection
• Infertility
• Miscarriage

People with untreated hypothyroidism are at increased risk for:

• Giving birth to a baby with birth defects
• Heart disease because of higher levels of LDL ("bad") cholesterol
• Heart failure

People treated with too much thyroid hormone are at risk for angina or heart attack, as well as osteoporosis (thinning of the bones).

Call your health care provider if you have symptoms of hypothyroidism (or myxedema).

If you are being treated for hypothyroidism, call your doctor if:

• You develop chest pain or rapid heartbeat
• You have an infection
• Your symptoms get worse or do not improve with treatment
• You develop new symptoms

There is no prevention for hypothyroidism.

Screening tests in newborns can detect hypothyroidism that is present from birth (congenital hypothyroidism).

Myxedema; Adult hypothyroidism

Fatourechi V. Subclinical hypothyroidism: an update for primary care physicians. Mayo Clin Proc. 2009;84(1):65-71.

Ladenson P, Kim M. Thyroid. In: Goldman L and Ausiello D, eds. Goldman: Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders; 2007:chap 244.

Vaidya B, Pearce SH. Management opf hypothyroidism in adults. BMJ. 2008;28;337:a801.

Updated by: Deborah Wexler, MD, Assistant Professor of Medicine, Harvard Medical School, Endocrinologist, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.