Structure and Function of the Endocrine System in Dogs

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:

• The hypothalamus is located at the base of the brain.
• The pituitary gland is located on the base of the brain and is attached to the hypothalamus via a stalk-like structure.
• The thyroid gland is located in the neck, below the larynx (voice box).
• Two parathyroid glands are located in the neck, closely associated with the thyroid gland.
• Two adrenal glands are located in the abdominal cavity directly in front of the
  kidneys.
• The gastrointestinal (GI) tract is located in the abdominal cavity.
• The pancreas is located in the forward part of the abdominal cavity, behind the liver and stomach.
• The liver is in the front of the abdomen, just behind the diaphragm and below the stomach.
• The ovaries are located in the middle part of the abdominal cavity near the kidneys.
• The testes are located in the scrotum.

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.

• The hypothalamus produces several hormones that tell the pituitary gland to secrete its hormones. The hypothalamus also produces antidiuretic hormone (ADH).
• The endocrine disorders associated with the pituitary gland fall into two types: under production of hormones (hypofunction of the gland), and excessive production of hormones (hyperfunction of the gland).

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.

• There are a number of common disorders associated with the thyroid gland.
• Parathyroid gland diseases are uncommon in the dog, and may reflect either hypofunction or hyperfunction of the parathyroids.

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.

• There are several endocrine disorders of the pancreas.
• The adrenal glands produce several hormones, but the most common disorders of this gland result in changes in cortisol levels. Cortisol is the cortisone hormone.

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.

• Other less common endocrine disorders involve changes in the amount of circulating fat (lipid) in the body, the underproduction or overproduction of red blood cells, and various functions of the reproductive system.

What Types of Diagnostic Tests Are Used to Evaluate the Endocrine System?

There are several tests that are helpful in evaluating the endocrine system.

• Blood tests that measure the amount of circulating hormone in the blood are the most common tests used to detect disorders of the endocrine system. Many different hormones can be measured in the blood, such as cortisol, thyroxine, ACTH, parathyroid hormone, growth hormone and insulin.
• In addition, blood tests have been developed that measure the response of endocrine glands to stimulating hormones. Most of these stimulating tests are timed blood tests. An initial blood sample is taken and the resting level of hormone is measured in that sample. Then a substance is injected into the body, and at some later time (usually within several hours), a second blood sample is taken. The hormone assay is repeated in the second sample to see if the hormone level has changed. Some stimulating tests involve giving different substances in sequence, with several blood samples taken at different intervals. The adrenal glands and thyroid glands are the most common endocrine glands evaluated by using stimulation tests.
• Routine serum biochemistry tests, a complete blood count, and a urinalysis can be used to detect clues that the endocrine system is diseased. For example, with Addison’s disease serum sodium is often low and serum potassium is high. With diabetes mellitus, serum or blood sugar is high. Many endocrine diseases also result in anemia. With Addison’s disease, Cushing’s disease, and diabetes insipidus the urine may be very watery or dilute. With diabetes mellitus, sugar is detected in the urine.
• Most endocrine glands do not show up on regular plain X-rays, but they may cause abnormalities in other organs that can be detected on X-ray. For example, the liver is often enlarged with Cushing’s disease and sometimes with diabetes mellitus. Elevated calcium levels associated with hyperparathyroidism may result in the formation of kidney or bladder stones that are visible on X-rays.
• CT scans and MRIs are very useful in evaluating the endocrine glands. They can often detect enlargement of the glands, distortion in their shape, and changes in their location and size. They provide valuable information about the structure of the glands, but they do not give much information on the function of the glands.
• Radioisotope scans are available for some endocrine glands, such as the thyroid gland. These tests involve the injection of radioactive materials that are taken up by the thyroid gland. These scans provide information about the location and size of the gland, as well as the function of the gland.
• Fine needle aspirates and biopsies are sometimes performed on certain endocrine glands. These tests are not usually done on the hypothalamus or pituitary gland, but may be considered for the thyroid gland, liver and kidney. They may also be performed on an enlarged parathyroid gland. Biopsies of the pancreas and adrenal gland must usually be performed during a surgical exploration of the abdomen and must be done cautiously.
• Some endocrine diseases are easy to diagnose, while others can be very difficult to confirm. Some endocrine disorders are not diagnosed until multiple tests are performed, sometimes over a period of time.