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Below is information about the structure and function of the canine endocrine system. We will tell you about the general structure of endocrine system, how it works in dogs, common diseases that affect the endocrine system and common diagnostic tests performed to evaluate the endocrine system in dogs.
What Is the Endocrine System?
The endocrine system is composed of several different types of glands and organs that produce the hormones of the dog’s body. A hormone is a chemical that is secreted by a gland in one area of the body and is carried by the bloodstream to other organs in the body, where it exerts some effect. Most hormones regulate the activity or structure of their target organs. The overall effect of the endocrine system is to regulate, coordinate and control many different bodily functions. The endocrine system includes the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands, part of the gastrointestinal tract, pancreas, kidneys, liver, ovaries and testes.
Where Is the Endocrine System Located in Dogs?
The endocrine system is scattered throughout the body as follows:
What Is the General Structure of the Canine Endocrine System?
The endocrine system is made up of a collection of glands distributed throughout the body. The endocrine glands produce hormones and secrete them directly into the internal environment where they are transmitted via the bloodstream. Hormones produce certain effects at different points in the body. Some endocrine glands are directly under the control of the pituitary gland. For example, the adrenal gland is controlled by the pituitary hormone, adrenocorticotropic hormone (ACTH). ACTH causes the adrenal glands to produce cortisone (cortisol), which is also a hormone. Other endocrine glands respond directly or indirectly to concentrations of substances in the blood, such as the insulin-secreting cells of the pancreas responding to the sugar concentration in the blood.
What Are the Functions of the Endocrine System in Dogs?
The major function of the endocrine system is to regulate numerous bodily functions, using specific hormones as messengers. Some hormones affect nearly all cells, while others regulate and affect only a single organ. Hormones act by regulating cell metabolism, by changing or maintaining enzyme activity in receptor cells, and by controlling growth and development, metabolic rate, sexual rhythms and reproduction.
The amount of hormone produced at any one time is controlled by feedback mechanisms. These feedback mechanisms are interactions between the endocrine glands, the blood levels of the various hormones, and certain activities of the target organ. For example, when the pituitary gland increases the secretion of ACTH, the increased levels are detected by the adrenal gland, and the end result is more production of cortisone hormone by the adrenal glands. As the cortisol levels rise in the bloodstream, the hypothalamus eventually detects these higher levels and sends a message to the pituitary gland. The pituitary gland then turns down its own production of ACTH. As the ACTH levels in the bloodstream subsequently fall, the adrenal gland decreases its production of cortisol to a normal level again. This is called a negative feedback loop.
What Are Common Diseases of the Endocrine System in Dogs?
Diseases of the endocrine system can arise with either overproduction or underproduction of hormones. There are numerous diseases of the endocrine system in dogs.
Diseases of the hypothalamus usually result in decreased function of the pituitary gland. The end result is less secretion of several pituitary hormones such as growth hormone, ACTH, or thyroid stimulating hormone.
Diabetes insipidus is a disease where the kidneys are unable to retain water because of a lack of antidiuretic hormone. Animals with diabetes insipidus, also known as water diabetes, are profoundly thirsty and urinate excessive amounts.
Undersecretion of pituitary growth hormone (GH) is not very common in the dog, but can occur in both puppies and adult dogs. Insufficient production of growth hormone in puppies results in dwarfism. When the production of GH is abnormally low in the adult dog, hair loss is the major symptom.
Overproduction of growth hormone causes a disorder called acromegaly. Acromegaly in the dog usually develops as a side effect to the long-term administration of progesterone drugs, but can also develop from a pituitary tumor. Affected dogs develop blunt and broad faces with excessive skin folds on the face and neck. They may develop enlargement of the abdomen, lethargy, weight gain and neurologic signs.
Hyperthyroidism arises when the thyroid gland becomes overactive and excessive amounts of the hormone thyroxine (T4) are released. Too much thyroxine in the body causes multiple clinical signs, including weight loss, increased appetite, vomiting, diarrhea, high blood pressure and sometimes, excessive activity. Hyperthyroidism is uncommon in the dog and most often arises from a cancerous tumor of the thyroid gland.
Hypothyroidism is a common disorder of dogs. It may arise with immune destruction of the thyroid gland. Some breeds of dogs are predisposed to this condition. With hypothyroidism the thyroid gland does not produces normal amounts of T4. Hypothyroid dogs often become sluggish and gain weight. They seek out warm places, have thin hair coats, and may develop neurologic and other signs.
Undersecretion of parathyroid hormone is called hypoparathyroidism. This condition may develop in young dogs, and may be due to immune destruction of the glands. Because parathyroid hormone is needed to maintain normal calcium levels in the body, hypoparathyroid dogs exhibit signs associated with low calcium. Signs include seizures, muscle twitching and tremors, trouble walking and weakness.
Oversecretion of parathyroid hormone, or hyperparathyroidism, also results in abnormal calcium levels in the body. This condition may arise with either benign or cancerous tumors of the gland. Calcium levels in the body become very elevated, and may result in kidney damage with increased urination, vomiting, loss of appetite, lethargy and muscle weakness.
Diabetes mellitus (or sugar diabetes) is an important disease of the endocrine portion of the pancreas. This common disorder of dogs arises with underproduction or inappropriately low secretion (release) of insulin. Insulin is the hormone that controls blood sugar and the usage of blood sugar by various organs in the body. Inadequate production of insulin causes the blood sugar to increase. Signs associated with elevated blood sugar include increased thirst and urination, weight loss despite a normal appetite, muscle weakness and development of cataracts.
Insulinomas are insulin-secreting tumors of the pancreas. Excessive amounts of insulin cause profound hypoglycemia (low blood sugar), and this often results in weakness, disorientation and seizures.
Other hormone secreting tumors may also develop in the pancreas, but they are rare in the dog.
The most common disease of the adrenal gland involves the overproduction of cortisol, also known as hyperadrenocorticism (hypercortisolism) or Cushing’s disease. Cushing’s disease is usually seen in middle aged to older dogs and often arises secondary to an overproduction of the hormone ACTH by the pituitary gland. A tumor of the adrenal gland may also result in too much cortisol secretion. Excessive drinking, urinating, increased appetite, panting, hair loss and a pot-bellied appearance are typical signs of too much cortisol production.
A less common disease of the adrenal gland is hypoadrenocorticism or Addison’s disease. Addison’s disease is seen more commonly in dogs than cats and is caused by a deficiency of two hormones, cortisone and aldosterone. Aldosterone regulates sodium and potassium levels in the body. Dogs with Addison’s disease are often weak, have low heart rates, vomiting, diarrhea, low blood sugar, and may be collapsed.
A tumor of the adrenal gland, called a pheochromocytoma, is a rare cause of high blood pressure in the dog. This tumor causes the overproduction of epinephrine hormone in the dog. It occurs primarily in older dogs.
What Types of Diagnostic Tests Are Used to Evaluate the Endocrine System?
There are several tests that are helpful in evaluating the endocrine system.