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.