Primary hypothyroidism is a condition in which a defect in the thyroid gland leads to reduced production of thyroid hormone.

The thyroid gland is an important organ that regulates metabolism. It is located in the front of the neck just below the voice box (larynx). The thyroid gland releases two forms of thyroid hormone – thyroxine (T4) and triiodothyronine (T3). The thyroid gland, along with the pituitary gland and hypothalamus in the brain, usually controls how much of these hormones are produced.

Primary hypothyroidism is when the thyroid cannot make the hormones T3 and T4 because of a problem with the gland itself. In the U.S., the most common cause is destruction of the thyroid gland by the immune system. This condition is called Hashimoto's thyroiditis. Not having enough iodine in the diet is a rare cause of hypothyroidism in the U.S.

Other causes of primary hypothyroidism include:

• Certain drugs such as lithium
• Radiation exposure to the neck
• Radioactive iodine used for treatment of hyperthyroidism (overactive thyroid)
• Special x-ray dyes
• Surgical removal of part or all of the thyroid gland

Some women develop hypothyroidism after pregnancy (often referred to as “postpartum thyroiditis"). In other cases, the cause of hypothyroidism is unknown.

Problems with the pituitary gland and hypothalamus may also cause the thyroid gland to produce too little thyroid hormone. This condition is called secondary hypothyroidism.

Risk factors for hypothyroidism include:

• Age (older than 50)
• Female gender
• Obesity
• Thyroid surgery
• X-ray or radiation treatments to the neck

Many of the symptoms of hypothyroidism also occur with a number of other conditions and problems.

Primary hypothyroidism affects the whole body and may cause a variety of symptoms. The body's normal rate of functioning slows, leading to mental and physical sluggishness. Symptoms may vary from mild to severe. The most severe form is called myxedema coma and is a medical emergency.

Early symptoms:

• Cold intolerance
• Constipation
• Depression
• Fatigue
• Muscle or joint pain
• Paleness
• Thin, brittle fingernails
• Thin, brittle hair
• Weakness
• Weight gain

Late symptoms:

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

Physical examination may reveal a smaller than normal gland. However, sometimes the gland is normal in size or even enlarged (goiter). Other signs include:

• Coarse facial features
• Firm swelling of the arms and legs
• Loss of the edges of the eyebrows
• Low blood pressure
• Low temperature
• Pale, yellow, and dry skin
• Slow heart rate
• Slow muscle relaxation when reflexes are tested
• Thin, brittle hair

A chest x-ray sometimes shows an enlarged heart.

Laboratory tests to determine thyroid function include:

• Free T4 or free thyroxine index (low)
• Free T4 test (low)
• Serum thyroid stimulating hormone (TSH) (high)

Other problems found 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 (T4) is the most commonly used medication. However a form of T3 is sometimes used together with thyroxine. A combination of T4 and T3 is also available.

Most people feel their best when TSH level is brought into the 1 - 2 mcIU/mL range. People get the lowest dose that effectively relieves their symptoms and brings their blood tests into the normal range.

Life-long therapy is needed. The condition will come back if therapy is interrupted. You must keep taking your medication even when your symptoms go away.

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

• Heat intolerance
• Rapid weight loss
• Restlessness
• Sweating

Myxedema coma is treated with intravenous thyroid replacement and steroid medications. Some people may need supportive therapy (oxygen, breathing assistance, and fluid replacement).

With early treatment, the condition can be completely controlled. However, the condition will return if you do not continue to take your medication.

Myxedema coma can result in death.

Myxedema coma, the most severe form of hypothyroidism, is rare. It may be caused by:

• Certain medications
• Exposure to cold
• Illness
• Infection

Symptoms and signs of myxedema coma include:

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

Other complications include:

• Depression
• Heart disease
• Increased risk of infection
• Infertility
• Miscarriage
• Pituitary tumors

Complications that can occur with too much thyroid hormone replacement include:

• Atrial fibrillation
• Osteoporosis
• Symptoms of hyperthyroidism

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

Also call if you experience these symptoms after beginning thyroid replacement therapy:

• Excessive sweating
• Heat intolerance
• Rapid heart rate
• Rapid weight loss
• Restlessness
• Symptoms of hyperthyroidism

Primary hypothyroidism is preventable by supplemental iodine in areas where iodine in the food supply is low. Otherwise, the condition is not preventable.

Being aware of your risk may allow early diagnosis and treatment. Some experts advocate TSH testing in certain high risk groups (e.g., women older than 50 years).

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, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders; 2007:chap 244.