The thyroid gland is located in the front of the neck just below the voice box (larynx). It releases hormones that control metabolism . These hormones are thyroxine (T4 ) and triiodothyronine (T3 ).

The release of T3 and T4 is controlled by the pituitary gland and the hypothalamus in the brain. Thyroid disorders may be caused by defects in the thyroid gland itself. They may also be caused by abnormalities of the pituitary or hypothalamus.

An underactive thyroid gland (hypothyroidism) may affect all body functions. The most severe form, called myxedema coma, is a medical emergency.

The most common cause of hypothyroidism is Hashimoto's thyroiditis , in which the immune system attacks the thyroid gland. A less common cause of hypothyroidism is when the pituitary gland does not release a hormone to stimulate the thyroid gland (secondary hypothyroidism ).

Other causes include:

• Congenital (birth) defects
• Inflammatory conditions
• Radiation to the gland
• Surgical removal of the thyroid gland

Risk factors include:

• Age over 50 years
• Exposure of the neck to x-ray or radiation treatments
• Female gender
• Obesity
• Thyroid surgery

Early symptoms:

• Abnormal menstrual periods
• Cold intolerance
• Constipation
• Depression
• Fatigue
• Joint or muscle pain
• Paleness
• Thin and brittle hair
• Thin, brittle fingernails
• Weakness
• Weight gain (unintentional)

Late symptoms:

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

Other symptoms that can occur with this disease:

• Absent menstruation
• Ankle, feet, and leg swelling
• Appetite loss
• Delayed growth of or missing teeth
• Drowsiness
• Dry hair
• Hair loss
• Joint stiffness
• Overall swelling
• Muscle pain
• Muscle spasms (cramps)
• Short stature
• Uncoordinated movement

A physical examination will show:

• Brittle nails
• Coarse facial features
• Firm swelling of the arms and legs
• Low blood pressure
• Low temperature
• Mental slowing
• Muscle atrophy
• Pale, yellow skin
• Separated sutures
• Slow heart rate
• Slow relaxation of muscles during reflex tests
• Thin and brittle hair

A chest x-ray may show an enlarged heart.

Laboratory tests to determine thyroid function include:

• T4 test
• Serum TSH

Other findings on lab tests may include:

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

Patients will need lifelong therapy. You must continue taking your medication even when your symptoms go away. After the medication dose is determined, your thyroid hormone levels should be monitored every year.

After you have begun taking replacement therapy, report to your health care provider any symptoms of increased thyroid activity (hyperthyroidism ) such as:

• Rapid weight loss
• Restlessness
• 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.

With treatment, most people will return to normal. However, you will need to take medication 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

Call your health care provider if:

• You have symptoms of hypothyroidism (or myxedema)
• 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. However, screening tests in newborns can detect congenital hypothyroidism.

AACE Thyroid Task Force. American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment Of Hyperthyroidism and Hypothyroidism. Endocr Pract. 2002;8 (6).

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