Secondary hypothyroidism is a condition in which the activity of the thyroid gland is decreased, due to failure of the pituitary gland or hypothalamus.

The thyroid gland is an important organ of the endocrine system. It is located in the front of the neck just below the voice box.

The thyroid gland releases the hormones thyroxine (T4) and triiodothyronine (T3), which control body metabolism. It also releases calcitonin, which plays a role in calcium balance and will not be discussed here.

The release of T3 and T4 by the thyroid gland is controlled by a system involving the pituitary gland and the hypothalamus (structures in the brain). Lowered levels of these thyroid hormones result in increased levels of hormones from the pituitary and hypothalamus. The reverse is also true -- when levels of thyroid hormones rise, hormones from the pituitary gland and hypothalamus fall. This helps keep hormone levels balanced.

Thyroid disorders may be caused by defects in the thyroid gland, and the disruption of the control system in the pituitary and hypothalamus.

Overproduction of T3 and T4 hormones is a condition called hyperthyroidism. Underproduction of these hormones is known as hypothyroidism.

Secondary hypothyroidism is due to a failure of the pituitary gland to release thyroid stimulating hormone (TSH) or thyrotropin releasing hormone (TRH). This is usually caused by a tumor in the area of the pituitary or hypothalamus. Or, it can be caused by radiation to the brain.

Excessive blood loss during labor and delivery can cause low blood flow and infection of the pituitary gland (Sheehan syndrome). Rarely, certain illnesses can damage the pituitary gland by swelling (inflammation) or creating iron deposits.

Risk factors for secondary hypothyroidism include:

• Age (over 50 years old)
• Gender (female)
• History of pituitary or hypothalamic dysfunction (including having received radiation to the pituitary or hypothalamus)

Hypothyroidism may cause a variety of symptoms and can affect all body functions. The body's normal rate of functioning slows, causing mental and physical sluggishness.

Symptoms vary from mild to severe. The most severe form is called myxedema. This is a medical emergency and can lead to coma and death.

Early symptoms:

• Brittle fingernails
• Coarseness, thinning of hair
• Cold intolerance
• Constipation
• Depression
• Fatigue
• Joint or muscle pain
• Weakness
• Weight gain

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

A physical exam usually reveals a small thyroid gland. Other signs include:

• Low blood pressure
• Low temperature
• Slow heart rate

A chest x-ray may reveal an enlarged heart.

Laboratory tests to determine thyroid function include:

• Free T4 test
• Serum TSH
• Total T3

Other lab test findings may include:

• Increased cholesterol levels
• Increased liver enzymes
• Increased serum prolactin
• Lack or excess of other pituitary hormones
• Low serum sodium

An MRI of the pituitary may be done to look for a tumor.

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 effective dose that returns thyroid function to normal. Life-long therapy may be necessary. You must keep taking medication even when your symptoms disappear.

After replacement therapy has begun, report any symptoms of increased thyroid activity (hyperthyroidism), such as:

• Rapid weight loss
• Restlessness
• Sweating

If you gained weight from low thyroid activity, a high-fiber, low-calorie diet and moderate activity will help relieve constipation and promote weight loss.

In people who also have an underactive adrenal gland (hypoadrenalism), steroid replacement must be started before thyroid replacement.

Patients who have hypothyroidism caused by a pituitary tumor may need surgery. However, surgery may not cure the hypothyroidism. Patients still may need thyroid replacement.

Myxedema coma is treated by intravenous (IV) thyroid replacement and steroid therapy. Some people may need oxygen, breathing assistance, fluid replacement, and intensive care nursing.

With early treatment, you should return to normal. However, the condition can return if you do not keep taking your medication.

Myxedema coma can result in death.

Myxedema coma, the most severe form of hypothyroidism, is rare. It may be brought on by an infection, illness, exposure to cold, or certain medications.

Symptoms and signs of myxedema coma include:

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

Other complications of hypothyroidism include:

• Adrenal crisis
• Abnormalities in the unborn baby (fetal abnormalities)
• Heart disease
• Infertility

Call your health care provider if you have:

• Breast discharge
• Chest pain
• Headache
• Rapid heartbeat
• Rapid weight loss
• Restlessness
• Sweating
• Symptoms of hypothyroidism
• Vision loss

This condition may not be preventable. Awareness of risk may allow early diagnosis and treatment.

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.