Understanding the endocrine gland system of a dog is necessary to understanding Cushing’s disease. The endocrine system delivers messages, in the form of hormones, to every part of the dog, by means of the blood stream. The pituitary gland, located at the base of the dog’s brain, and the two adrenal glands, located near the kidneys, are the endocrine glands in charge of controlling the levels of cortisol. Cortisol is a hormone that helps maintain the homeostasis of the body, by converting protein into energy, improving fight or flight instinct, supporting the immune system, memory, and general coping with psychological and physiological stresses.
The pituitary gland produces the hormone adrenocorticotrophic (ACTH), which modulates the release of cortisol, by sending a message to the adrenal glands, about whether or not the body is under stress, and to what degree. The more stress, the more cortisol the adrenal glands are told to produce. This process is integral to the balance of the animal’s entire system. There are three types of Cushing’s disease; pituitary dependent, adrenal dependent, and iatrogenic Cushing’s.
Pituitary Dependent Cushing’s Disease
Pituitary dependent Cushing’s disease is caused by small tumors on the pituitary gland itself. These small tumors, known as microadenomas, prevent the pituitary gland from understanding that the blood cortisol levels are high. Therefore the adrenal glands continuously produce cortisol, effectively poisoning the blood.
Physically, a dog with pituitary dependent Cushing’s disease, tends to have very large adrenal glands, due to constant work. Occasionally, the tumors may metastasize to over one centimeter, and may cause other problems, such as blindness, not associated directly with Cushing’s disease.
Adrenal Dependent Cushing’s Disease
Adrenal dependent Cushing’s disease is caused by similar tumors to the pituitary dependent Cushing’s. However, they directly impede the cortisol level receptors. The adrenal gland no longer understands, or just ignores the information from the pituitary gland, pertaining to cortisol release. These tumors are adenomas, and half the time are benign, but the other half of the time they are malignant, or adenocarcinomas.
A dog with adrenal dependent Cushing’s disease typically has tumors on one adrenal gland, causing the overproduction of cortisol, resulting in enlargement. If the other adrenal gland remains free of tumors, it tends to shrink from underproduction to compensate.
Iatrogenic Cushing’s Disease
Iatrogenic Cushing’s is caused by extended exposure to powerful external steroids. If a dog suffers from chronic conditions, such as allergies, and is administered external steroids, such as prednisone, in high doses for an extended period of time, the adrenal glands can no longer control the hormone levels in the blood and then will atrophy.
A dog with iatrogenic Cushing’s disease typically has very small atrophied adrenal glands from lack of work.
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