Hypothyroidism is a condition in which the thyroid gland fails to produce enough thyroid hormone.

The thyroid gland, located in the front of the neck just below the larynx, secretes hormones that control metabolism. These hormones are thyroxine (T4) and triiodothyronine (T3).

The secretion of T3 and T4 is controlled by the pituitary gland and the hypothalamus, which is part of the brain. Thyroid disorders may result not only from defects in the thyroid gland itself, but also from abnormalities of the pituitary or hypothalamus.

Hypothyroidism, or underactivity of the thyroid gland, may cause a variety of symptoms and may affect all body functions. The body's normal rate of functioning slows, causing mental and physical sluggishness. The symptoms may vary from mild to severe. The most severe form, called myxedema coma, is a medical emergency.

The most common cause of hypothyroidism is Hashimoto's thyroiditis, a disease of the thyroid gland where the body's immune system attacks the gland. Failure of the pituitary gland to secrete a hormone to stimulate the thyroid gland (secondary hypothyroidism) is a less common cause of hypothyroidism. Other causes include congenital (birth) defects, surgical removal of the thyroid gland, irradiation of the gland, or inflammatory conditions.

Risk factors include age over 50 years, female gender, obesity, thyroid surgery, and exposure of the neck to X-ray or radiation treatments.

Early symptoms:

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

Late symptoms:

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

Additional symptoms that may be associated with this disease:

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

A physical examination reveals delayed relaxation of muscles during tests of reflexes. Other findings may include pale, yellow skin, thin and brittle hair, coarse facial features, brittle nails, firm swelling of the arms and legs, and mental slowing. Vital signs may show slow heart rate, low blood pressure, and low temperature.

A chest x-ray may show an enlarged heart.

Laboratory tests to determine thyroid function include:

• T4 test (low)
• T3 test
• Serum TSH (high in primary hypothyroidism, low or low-normal in secondary hypothyroidism)

Additional laboratory abnormalities may include:

• Increased cholesterol levels
• Increased liver enzymes
• Increased serum prolactin
• Low serum sodium
• A complete blood count (CBC) that shows anemia

The purpose of treatment is to replace the deficient thyroid hormone. Levothyroxine is the most commonly used medication. The lowest dose effective in relieving symptoms and normalizing the TSH is used. Life-long therapy is needed. Medication must be continued even when symptoms subside. Thyroid hormone levels should be monitored yearly after a stable dose of medication is determined.

After replacement therapy has begun, report any symptoms of increased thyroid activity (hyperthyroidism) such as restlessness, rapid weight loss, and 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 hormones replacement and steroid therapy. Supportive therapy (oxygen, assisted ventilation, fluid replacement) and intensive-care nursing may be indicated.

With treatment, return to the normal state is usual. Life-long medication is needed. 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 an individual with untreated hypothyroidism. Symptoms and signs of myxedema coma include unresponsiveness, decreased breathing, low blood pressure, low blood sugar, and below normal temperature.

Other complications are heart disease, increased risk of infection, infertility, and miscarriage.

Call your health care provider if signs of hypothyroidism (or myxedema) are present.

Call your health care provider if chest pain or rapid heart beat occur, infection occurs, symptoms worsen or do not improve with treatment, or new symptoms develop.

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).