Structure and Function of the Endocrine System in Cats

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What Is the Endocrine System?

The endocrine system is composed of several different types of glands and organs that produce the hormones of the 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?

The endocrine system is scattered through out 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).
  • There are two parathyroid glands located in the neck, closely associated with the thyroid gland.
  • There are two adrenal glands 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 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?

    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?

    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).

    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. Diabetes insipidus is extremely rare in the cat.

  • The endocrine disorders associated with the pituitary gland fall into two types: underproduction of hormones (hypofunction of the gland), and excessive production of hormones (hyperfunction of the gland).

    Undersecretion of pituitary growth hormone (GH) in young kittens is theoretically possible, but has not been well documented. Insufficient production of growth hormone in puppies results in dwarfism, but a comparable disorder is not known to occur in kittens.

    Overproduction of growth hormone causes a disorder called acromegaly. Acromegaly in the cat usually develops from a pituitary tumor. Affected cats are usually severely ill with signs that reflect poorly controlled diabetes mellitus, heart disease and kidney failure.

  • There are a number of common disorders associated with the thyroid gland.

    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 and excessive vocalization. Hyperthyroidism is common in older cats and may be related to nutritional or environmental factors.

    Hypothyroidism is a rare disorder of cats. It arises most often after the thyroid glands have been surgically removed to treat hyperthyroidism. With hypothyroidism the thyroid gland does not produces normal amounts of T4. Hypothyroid cats often become sluggish, gain weight, and may become constipated. They have poorly groomed hair coats, with dry and flaky skin.

  • Parathyroid gland diseases are uncommon in the cat, and may reflect either hypofunction or hyperfunction of the parathyroids.

    Undersecretion of parathyroid hormone is called hypoparathyroidism. This condition may develop in young or adult cats, and may be due to immune destruction of the glands. Hypoparathyroidism may also develop in the cat if these glands were inadvertently removed at the same time the thyroid glands were removed (to treat hyperthyroidism). Because parathyroid hormone is needed to maintain normal calcium levels in the body, hypoparathyroid cats 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, and is most often seen in older cats. 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.

    Diabetes mellitus (or sugar diabetes) is an important disease of the endocrine portion of the pancreas. This common disorder of cats 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, and muscle weakness.

    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. These tumors are extremely rare in the cat.

  • 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. The disease does not occur in the cat as often as it does in dogs. Cushing's disease is usually seen in middle aged to older cats 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. Affected cats may have very nonspecific signs, such as lethargy, decreased appetite, weight loss, dehydration, weakness and vomiting.

    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. Cats with Addison's disease are often young, and have nonspecific clinical signs, such as weakness, lethargy, vomiting, diarrhea and blood in the bowel movements.

    A tumor of the adrenal gland, called a pheochromocytoma, is an extremely rare cause of high blood pressure in the cat. This tumor causes the over production of norepinephrine hormone in the cat. It occurs primarily in older cats.

  • Other endocrine disorders involve 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 hyperthyroidism, several liver and muscle enzyme tests may be elevated. With diabetes mellitus, serum or blood sugar is high. Many endocrine diseases also result in anemia. With Cushing's disease the urine may be very watery or dilute. With diabetes mellitus and acromegaly, 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 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. Because some endocrine diseases of cats cause very vague symptoms, they can often be overlooked.

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