Definition of Hypothyroidism - secondary:

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

Causes, incidence, and risk factors:

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)