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Introduction to Hormonal Disorders of Dogs

By Deborah S. Greco, DVM, PhD, DACVIM, Nestle Purina PetCare ; David Bruyette, DVM, DACVIM, VCA West Los Angeles Animal Hospital ; Robert J. Kemppainen, DVM, PhD, Department of Anatomy, Physiology & Pharmacology, College of Veterinary Medicine, Auburn University ; Mark E. Peterson, DVM, DACVIM, Animal Endocrine Clinic ; Robert C. Rosenthal, DVM, PhD, DACVIM (Small Animal, Oncology), DACVR (Radiation Oncology)

Hormones are chemical messengers that have many different functions. The effects of hormones in the body are wide-ranging and varied. Some familiar examples of hormones include insulin, which is important in the development of diabetes, and estrogen and progesterone, which are involved in the female reproductive cycle.

The endocrine system consists of a group of tissues that release hormones into the bloodstream for travel to other parts of the body (see Table: Major Hormones). Most endocrine tissues are glands (such as the thyroid gland) that release hormones directly into small blood vessels within and around the tissue. Several important hormones are released from tissues other than glands, such as the heart, kidney, and liver. Some hormones act only on a single tissue, while others have effects on virtually every cell in the body. Hormones are present in the blood in very small quantities, so laboratory tests done to measure hormone levels must be very sensitive.

The major endocrine glands in the dog

Major Hormones

Endocrine Gland

Hormone(s) Produced

Function

Pituitary gland (anterior lobe)

Corticotropin (adrenocorticotropic hormone)

Stimulates the production and secretion of hormones by the adrenal cortex

Growth hormone

Promotes growth of the body and influences the metabolism of proteins, carbohydrates, and lipids

Follicle-stimulating hormone

Stimulates the growth of follicles in the ovaries and induces the formation of sperm in the testes

Luteinizing hormone

Stimulates ovulation and the development of the corpus luteum in the female and the production of testosterone by the testes in the male

Prolactin

Stimulates the mammary glands to secrete milk

Thyroid-stimulating hormone

Stimulates the production and secretion of thyroid hormones by the thyroid gland

Pituitary gland (posterior lobe)

Antidiuretic hormone; also known as arginine vasopressin

Causes the kidneys to conserve water by concentrating the urine and reducing urine volume; also has lesser role in regulating blood pressure

Oxytocin

Stimulates the contraction of smooth muscle of the uterus during labor and facilitates ejection of milk from the breast during nursing

Parathyroid glands

Parathyroid hormone

Raises the blood calcium concentration by promoting absorption of calcium by the intestine, mobilizing calcium salts from bones, and increasing the ability of the kidney to recover calcium from urine; also lowers phosphate by enhancing its excretion by the kidneys

Thyroid glands

Thyroid hormones (T3 and T4 )

Increase the basal metabolic rate; also regulate protein, fat, and carbohydrate metabolism

Calcitonin

Participates in calcium and phosphorus metabolism; tends to have the opposite effects of parathyroid hormone

Adrenal glands

Aldosterone

Helps regulate salt and water balance by retaining sodium (salt) and water and excreting potassium

Cortisol

Has widespread effects throughout the body; involved in the response to stress; active in carbohydrate and protein metabolism; helps maintain blood sugar level, blood pressure, and muscle strength

Epinephrine (adrenaline) and norepinephrine

Released in response to stress; stimulates heart action and increases blood pressure, metabolic rate, and blood glucose concentration; also raises blood sugar and fatty acid levels

Pancreas

Insulin

Lowers blood sugar level; affects the metabolism of sugar, protein, and fat

Glucagon

Raises blood sugar level, thus opposing the action of insulin

Ovaries

Estrogen

Controls female reproductive system, along with other hormones; responsible for promoting estrus and the development and maintenance of female secondary sex characteristics

Progesterone

Prepares the uterus for implantation of the fertilized egg, maintains pregnancy, and promotes development of the mammary glands

Testes

Testosterone

Responsible for the development of the male reproductive system and secondary male sexual characteristics

Development of Endocrine System Disease

The body monitors and adjusts the level of each hormone by using a feedback system specifically for that hormone. Hormones function to keep factors such as temperature and blood sugar (glucose) levels within certain ranges. Sometimes, pairs of hormones with opposite functions work together to keep body functions in balance.

Endocrine system diseases can develop when too much or not enough hormone is produced, or when normal pathways for hormones to be used and removed are disrupted. Signs can develop because of a problem in the tissues that are the source of the hormone, or because of a problem in another part of the body that is affecting the secretion or action of a particular hormone.

A tumor or other abnormal tissue in an endocrine gland often causes it to produce too much hormone. When an endocrine gland is destroyed, not enough hormone is produced. Diseases caused by overproduction or excess of a hormone often begin with the prefix hyper. For example, in hyperthyroidism, the thyroid gland produces too much thyroid hormone. Diseases caused by a lack or deficiency of a hormone often begin with the prefix hypo. For example, in hypothyroidism, the thyroid gland does not produce enough thyroid hormone.

In many cases, the abnormal gland not only overproduces hormone, it also does not respond normally to feedback signals. This causes hormone to be released in situations in which its levels would normally be reduced. Sometimes, the overproduction is caused by stimulation from another part of the body. Occasionally, a tumor outside the endocrine system can produce a substance similar to a hormone, causing the body to respond as though that hormone were being produced.

Diseases caused by not enough hormone secretion can also have multiple causes. Endocrine tissue can be destroyed by an autoimmune process, in which the body incorrectly identifies some of its own tissue as foreign and destroys the tissue cells. In early stages of tissue loss, the body may compensate by producing additional hormone from the remaining tissue. In these cases, signs of disease may be delayed until the tissue has been destroyed completely.

Disorders resulting in signs of reduced endocrine activity may also develop because tissues distant from the hormone source are disrupted. This can occur when the function of one hormone is to stimulate the production of a second hormone. For example, the pituitary gland secretes a hormone that stimulates the thyroid gland to secrete thyroid hormones. If the levels of the thyroid-stimulating hormone from the pituitary gland are abnormally low, the levels of thyroid hormones will also be low even if the thyroid gland is healthy. Another potential cause for reduced endocrine function is tissue loss caused by tumors that do not produce hormones themselves but compress or destroy the nearby endocrine gland.

Endocrine diseases and related conditions also result from changes in the response of tissues targeted by a hormone. An important example is type 2 diabetes mellitus, in which the body produces insulin but the cells no longer respond to it. This condition is often associated with obesity.

Treatment of Endocrine System Disease

Endocrine diseases caused by the presence of too much hormone may be treated surgically (tumor removal), by radiotherapy (such as the use of radioactive iodine to destroy an overactive thyroid gland), or with medication. Syndromes of hormone deficiency are often successfully treated by replacing the missing hormone, such as insulin injections to treat diabetes mellitus. Steroid and thyroid hormone replacements can usually be given orally.

Pets taking hormone replacement treatment must be monitored for adverse effects and periodically retested to make sure the dosage is correct. In some cases, such as after surgical removal of an endocrine tumor, the diseased gland will recover and hormone replacement will no longer be needed. However, most of the time, lifelong treatment is required.

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