Secondary hypothyroidism is a condition where the activity of the thyroid gland is decreased, due to failure of the pituitary gland.

The thyroid gland is an important organ of the endocrine system, located in the front of the neck just below the voice box. The gland secretes the hormones thyroxine (T4), triiodothyronine (T3), and calcitonin, which control body metabolism and regulate calcium balance.

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 pituitary and hypothalamic hormones. The reverse is also true -- when levels of the thyroid hormones rise, pituitary and hypothalamic hormones fall back. This helps keep levels appropriately balanced.

Since the thyroid gland is regulated by the pituitary gland and the hypothalamus, thyroid disorders may result not only from defects in the thyroid itself but also from the disruption of the control system in these other organs.

Thyroid disorders caused by overproduction of T3 and T4 are called hyperthyroidism, and underproduction of these hormones is known as hypothyroidism.

The cause of secondary hypothyroidism is failure of the pituitary gland to release thyroid stimulating hormone (TSH). This is usually caused by a tumor in the region of the pituitary. Rarely the cause is an infiltration of the pituitary by inflammatory cells from the immune system, or due to foreign substances (such as iron in hemochromatosis).

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, which is a medical emergency and can lead to coma and death.

Risk factors for secondary hypothyroidism include being over 50 years old, being female, and having a history of pituitary or hypothalamic dysfunction (including having received radiation to the pituitary or hypothalamus areas, even years before).

Early symptoms:

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

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

A physical exam usually reveals a small thyroid gland. Vital signs (temperature, pulse, rate of breathing, blood pressure) reveal a slow heart rate, low blood pressure, and low temperature.

A chest x-ray may reveal an enlarged heart.

Laboratory tests to determine thyroid function include:

• Free T4 test
• Total T3
• Serum TSH -- Results are generally low in secondary hypothyroidism because the pituitary is damaged. However, normal or even high values may be seen.

Additional laboratory abnormalities may include:

• Increased cholesterol levels
• Increased liver enzymes
• Increased serum prolactin
• Low serum sodium
• Low blood glucose
• A CBC that shows anemia
• Deficiency or excess of other pituitary hormones

Imaging will include an MRI of the pituitary to look for a tumor.

The purpose of treatment is to replace the deficient thyroid hormone. Levothyroxine is the most commonly used medication. The lowest effective dose that leads to normal thyroid function is used. Life-long therapy may be necessary. Medication must be continued even when symptoms disappear.

After replacement therapy has begun, report any symptoms of increased thyroid activity (hyperthyroidism), such as restlessness, rapid weight loss, and sweating.

A high-fiber, low-calorie diet and moderate activity will help relieve constipation and promote weight loss, if weight was gained during the time when thyroid activity was low.

In individuals with accompanying hypoadrenalism (underactive adrenal gland), steroid replacement must be started before thyroid replacement is begun.

In patients who have hypothyroidism caused by a pituitary tumor, surgery may be required. However, surgery may not cure the hypothyroidism, and thyroid replacement will still be needed.

Myxedema coma is treated by intravenous (IV) thyroid replacement and steroid therapy. Supportive therapy of oxygen, assisted ventilation, fluid replacement, and intensive care nursing may be indicated.

With early treatment, return to the normal state is usual. However, relapses will occur if the medication is not continued. 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:

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

• Heart disease
• Infertility
• Miscarriage in pregnant women
• Adrenal crisis, if thyroid replacement is begun prior to steroid treatment in hypoadrenal patients

Call your health care provider if signs of hypothyroidism are present, or if chest pain or rapid heartbeat occur.

Call your provider if restlessness, rapid weight loss, sweating, or other symptoms occur after beginning treatment for this disorder.

Call your provider if headache, visual loss, or breast discharge occur.

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).