Cushing’s disease is a form of Cushing’s syndrome, in which high cortisol levels cause debilitating physical, mental and hormonal symptoms.
Importance of cortisol in the body
Cortisol is a steroid hormone belonging to the glucocorticoid class of hormones. Among its many important physiological roles, it regulates blood pressure; limits the immune system’s response to inflammation; controls the body’s use of macronutrients through limiting the effects of insulin, and helps the body respond to stress.
When cortisol levels are too high — a condition known as hypercortisolism — patients experience harmful physical, mental, and hormonal changes.
Exogenous and endogenous Cushing’s syndrome
Cushing’s syndrome is the collection of all hypercortisolism disorders. One type of hypercortisolism, known as exogenous Cushing’s syndrome, is caused by too much external cortisol being added to the body. This occurs when a person regularly takes high doses of corticosteroids for an extended period. Prolonged corticosteroid use is often necessary to treat inflammatory diseases and ailments such as asthma, arthritis and joint or back pain. Generally prescribed steroid dosages raise cortisol levels in the body, which can cause symptoms of Cushing’s syndrome.
Forms of Cushing’s syndrome can also be caused by overproduction of cortisol within the body, known as endogenous Cushing’s syndrome. When this is caused by a tumor in the pituitary gland, the condition is called Cushing’s disease.
The pituitary gland is a small, pea-sized gland located at the base of the brain. In Cushing’s disease patients, a tumor causes this gland to release excess adrenocorticotropic hormone (ACTH). The most common type of tumor causing increased ACTH production is an adenoma, which is rarely cancerous.
The role of ACTH
A primary role of ACTH is to stimulate cortisol production in the outer portions of the adrenal glands, located above the kidneys, with one above each kidney. In healthy patients, when cortisol level in the bloodstream exceeds a threshold, cortisol binds to certain receptors to turn off the production of ACTH. This negative feedback loop does not function correctly in patients with Cushing’s disease, and high cortisol levels do not halt the production of ACTH.
High ACTH levels will not only directly trigger cortisol production in the adrenal glands, but will also cause the adrenal glands to grow, increasing cortisol production further. Patients with Cushing’s disease will show high levels of ACTH and cortisol in blood tests.
In addition to tumors on the pituitary gland, tumors that form elsewhere in the body can also trigger raised cortisol levels. Tumors on the adrenal gland for instance directly stimulate excess cortisol secretion, without changes in ACTH levels. In very rare cases, a growth may develop in an organ that does not produce cortisol or ACTH, such as the lungs, pancreas, or thyroid, and this tumor can produce excess levels of ACTH, significantly raising cortisol levels. These ACTH-producing tumors can easily be confused with Cushing’s disease in diagnoses based on blood tests.
In other Cushing’s syndrome disorders, ACTH will be within normal levels, so physicians usually take ACTH and cortisol levels into account when diagnosing the cause of the Cushing’s syndrome.
Is Cushing’s disease hereditary?
No clear environmental or hereditary causes have been identified, though a very small population of patients has demonstrated familial trends. This may be due to a hereditary predisposition to develop pituitary tumors.
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