Addison's disease: what is it?
Addison's disease is a rare endocrine, or hormonal, disorder that affects about one in 100,000 people. It occurs in all age groups and afflicts men and women equally. The disease is characterised by weight loss, muscle weakness, fatigue, low blood pressure, and sometimes darkening of the skin in both exposed and nonexposed parts of the body.
Addison's disease occurs when the adrenal glands do not produce enough of the hormone cortisol and in some cases, the hormone aldosterone. For this reason, the disease is sometimes called chronic adrenal insufficiency, or hypocortisolism.
What is cortisol?
Cortisol is normally produced by the adrenal glands, located just above the kidneys. It belongs to a class of hormones called glucocorticoids, which affect almost every organ and tissue in the body. Scientists think that cortisol has possibly hundreds of effects in the body. Cortisol's most important job is to help the body respond to stress.
Among its other vital tasks, cortisol:
- helps maintain blood pressure and cardiovascular function;
- helps slow the immune system's inflammatory response;
- helps balance the effects of insulin in breaking down sugar for energy; and
- helps regulate the metabolism of proteins, carbohydrates, and fats.
Because cortisol is so vital to health, the amount of cortisol produced by the adrenals is precisely balanced.
Like many other hormones, cortisol is regulated by the brain's hypothalamus and the pituitary gland, a bean-sized organ at the base of the brain. First, the hypothalamus sends 'releasing hormones' to the pituitary gland. The pituitary responds by secreting other hormones that regulate growth, thyroid and adrenal function, and sex hormones such as oestrogen and testosterone. One of the pituitary's main functions is to secrete adrenocorticotropin (ACTH), a hormone that stimulates the adrenal glands. When the adrenals receive the pituitary's signal in the form of ACTH, they respond by producing cortisol. Completing the cycle, cortisol then signals the pituitary to lower secretion of ACTH.
What is aldosterone?
Aldosterone belongs to a class of hormones called mineralocorticoids, also produced by the adrenal glands. It helps maintain blood pressure and water and salt balance in the body by helping the kidneys retain sodium and excrete potassium. When aldosterone production falls too low, the kidneys are not able to regulate salt and water balance, causing blood volume and blood pressure to drop.
What causes adrenal insufficiency?
Failure to produce adequate levels of cortisol, or adrenal insufficiency, can occur for different reasons. The problem may be due to a disorder of the adrenal glands themselves (primary adrenal insufficiency) or to inadequate secretion of ACTH by the pituitary gland (secondary adrenal insufficiency).
Primary adrenal insufficiency
Most cases of Addison's disease are caused by the gradual destruction of the adrenal cortex, the outer layer of the adrenal glands, by the body's own immune system. About 70 per cent of reported cases of Addison's disease are due to autoimmune disorders, in which the immune system makes antibodies that attack the body's own tissues or organs and slowly destroy them.
Adrenal insufficiency occurs when at least 90 per cent of the adrenal cortex has been destroyed. As a result, often both glucocorticoid and mineralocorticoid hormones are lacking. Sometimes only the adrenal gland is affected, as in idiopathic adrenal insufficiency; sometimes other glands also are affected, as in polyendocrine deficiency syndrome.
Polyendocrine deficiency syndrome is classified into 2 separate forms, referred to as type I and type II. Type I occurs in children, and adrenal insufficiency may be accompanied by underactive parathyroid glands, slow sexual development, pernicious anaemia, chronic candida infections, chronic active hepatitis, and, in very rare cases, hair loss.
Type II, often called Schmidt's syndrome, usually afflicts young adults. Features of type II may include an underactive thyroid gland, slow sexual development, and diabetes mellitus. About 10 per cent of patients with type II have vitiligo, or loss of pigment, on areas of the skin.
Scientists think that polyendocrine deficiency syndrome is inherited because frequently more than one family member tends to have one or more endocrine deficiencies.
Tuberculosis (TB) accounts for about 20 per cent of cases of primary adrenal insufficiency in developed countries. When adrenal insufficiency was first identified by Dr Thomas Addison in 1849, TB was found at autopsy in 70 to 90 per cent of cases. As the treatment for TB improved, however, the incidence of adrenal insufficiency due to TB of the adrenal glands has greatly decreased.
Less common causes of primary adrenal insufficiency are:
- chronic infections, mainly fungal infections;
- cancer cells spreading from other parts of the body to the adrenal glands;
- amyloidosis; and
- surgical removal of the adrenal glands.
Secondary adrenal insufficiency
This form of Addison's disease can be traced to a lack of ACTH, which causes a drop in the adrenal glands' production of cortisol but not aldosterone. A temporary form of secondary adrenal insufficiency may occur when a person who has been receiving a glucocorticoid hormone such as prednisone for a long time abruptly stops or interrupts taking the medication.
Glucocorticoid hormones, which are often used to treat inflammatory illnesses such as rheumatoid arthritis, asthma, or ulcerative colitis, block the release of both corticotropin-releasing hormone (CRH) and ACTH. Normally, CRH instructs the pituitary gland to release ACTH. If CRH levels drop, the pituitary is not stimulated to release ACTH, and the adrenals then fail to secrete sufficient levels of cortisol.
Another cause of secondary adrenal insufficiency is the surgical removal of benign, or noncancerous, ACTH-producing tumours of the pituitary gland (Cushing's disease). In this case, the source of ACTH is suddenly removed, and replacement hormone must be taken until normal ACTH and cortisol production resumes.
Less commonly, adrenal insufficiency occurs when the pituitary gland either decreases in size or stops producing ACTH. This can result from tumours or infections of the area, loss of blood flow to the pituitary, radiation for the treatment of pituitary tumours, or surgical removal of parts of the hypothalamus or the pituitary gland during neurosurgery of these areas.
Last Reviewed: 03 June 2007
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