A cat’s endocrine system is responsible for the balance of hormones and regulate each hormone’s level in the bloodstream. Its major function is to regulate the numerous bodily functions using hormones to communicate.
Hormones are chemical messengers that have a variety of different functions that regulate the activity or structure of their target organs. Some regulate cell metabolism, change or maintain enzyme activity in receptor cells, control cell growth and development, metabolic rate, sexual rhythms, and reproduction. All are important for life. Some familiar examples of hormones include insulin, dopamine, and cortisol and an imbalance can cause the development of diseases such as diabetes.
While powerful, the organs and glands of the endocrine are proportionally smaller than the rest of the organs in the body. They are distributed throughout the body and secretes hormones via the bloodstream.
Some glands are directly under the control of the pituitary gland, which secretes the pituitary hormone and adrenocorticotropic hormone (ACTH). For example, ACTH causes adrenal glands to produce cortisone (cortisol), another hormone. Other endocrine glands respond directly or indirectly to concentrations of substances in the blood, such as the pancreas secreting insulin in a response to the blood sugar concentration.
The amount of hormone produced at any given time is controlled by feedback mechanisms. These feedback mechanisms are interactions between the endocrine glands, the blood levels of various hormones, and certain activities of the target organ. For example, when the pituitary gland increases ACTH secretion, the increased levels are detected the by the adrenal gland, which responds with production of more cortisone hormones.
Reversing the increased hormone levels is called a negative feedback loop. Continuing the example, the hypothalamus will detect the increased cortisol levels in the bloodstream and sends a message to the pituitary gland. In turn, the pituitary gland will turn down its own production of ACTH. As ACTH levels subsequently fall, the adrenal gland will decrease its production of cortisol to a normal level again.
Related Organs and Glands
Endocrine glands are essentially identical among all mammals, both in structure and function. The main glands include the pituitary gland, thyroid gland, parathyroid glands, pancreas, adrenal glands, ovaries, and testes.
- Hypothalamus: It is located at the base of the brain and is the portion of the brain that maintains homeostasis, the body’s internal balance. It is the link between the endocrine and nervous system. The hypothalamus produces releasing and inhibiting hormones, which stop and start the production of other hormones throughout the body.
- Pituitary gland: This gland is located near the center and bottom of the brain. It produces a number of critical hormones that control many parts of the body, including several other endocrine glands. Because of its central role, it is often referred to as the “master gland”.
- Thyroid gland: It is a two-lobed gland in the neck, located in front of the trachea, the breathing airway, and below the Adam’s apple. It produces two iodine-containing hormones: triiodothyronine (T3) and thyroxine (T4). In general, hormones produced in the thyroid regulate metabolic rate.
- Parathyroid glands: These four small glands are typically located adjacent or within the thyroid gland. They secrete parathyroid hormone (PTH), which helps maintain normal calcium and phosphorous levels in the body necessary for proper bone development.
- Pancreas: It is an oblong, flattened gland located in the abdomen, adjacent to the stomach and small intestine. While it does also secrete digestive juices to aid in digestion, its function in the endocrine system is to maintain healthy glucose, or blood sugar, levels. If blood sugar is too high, the pancreas will secrete insulin, which moves the sugar from the blood into the cells for energy consumption. If blood sugar is too low, the pancreas releases glucagon, which releases sugar stored in the liver into the bloodstream.
- Adrenal glands: Mammals have two adrenal glands located on the top of each kidney. Essentially, each gland contains two separate endocrine organs: the adrenal cortex and the adrenal medulla.
- Adrenal cortex: This is the outer portion of the adrenal gland whose hormones are essential for life. It produces glucocorticoids (e,g, cortisol) that help the body control blood sugar, increase the burning of protein and fat, and respond to stressful stimuli. In addition, it produces mineralocorticoids (e.g. aldosterone), which control blood volume and aid in regulating blood pressure by helping the kidneys hold onto to sodium and water.
- Adrenal medulla: This is the inner portion of the gland. The adrenal medulla produces epinephrine, also known as adrenaline, which increases heart rate, opens airways to improve oxygen intake, and increases blood flow to the muscles. It will also secrete norepinephrine that is more related to maintaining normal activities.
- Testes: In males, this produces the hormone testosterone, which helps develop and maintain male sexual traits.
- Ovaries: The female reproductive glands produce estrogen and progesterone. These hormones are responsible for developing and maintain female sexual traits, as well as maintaining a pregnancy. Along with certain pituitary hormones, they control the estrus cycle.
Diseases of the Endocrine System
Diseases of the endocrine system typically arise due to the overproduction or underproduction of hormones.
- Hypothyroidism: This thyroid disorder results from too little T3 or T4. It most commonly leads to stunted growth and dwarfism. Symptoms include decreased energy, lethargy, weight gain, constipation, dry skin, and hair loss.
- Hyperthyroidism: This thyroid disorder results from too much T3 or T4. Common clinical signs include increased appetite, nervousness, excitability, vomiting after eating, and increased thirst and urination. Usually occur in older cats and may be related to nutritional or environmental factors.
- Diabetes: It is a disorder of the pancreas that causes an imbalance of blood sugar levels. This disorder occurs when the pancreas does not produce enough insulin or the body is resistant to the insulin in the blood. Without enough insulin, glucose cannot be moved through the metabolic process.
- Hyperinsulinism: It is caused by too much insulin and leads to hypoglycemia, low blood pressure. This most commonly occurs when owners inadvertently overdose their pets with insulin. Insulinoma, insulin-secreting pancreatic tumor, will also cause hypoglycemia. Symptoms include anxiety, weakness, and seizures.
- Diabetes insipidus: It is a disease where the kidneys are unable to retain water because of a lack of antidiuretic hormone. Also known as water diabetes, affected animals are profoundly thirsty and urinate excessive amounts. It is extremely rare in cats.
- Undersecretion of Acromegaly: This disorder is caused by overproduction of a growth hormone. It usually develops from a pituitary tumor. Affected cats fall severely ill with signs that reflect poorly controlled diabetes mellitus, heart disease, and kidney failure.
- Hypoparathyroidism: This condition is caused by the undersecretion of the parathyroid hormone. It may develop in young or adult cats and may be due to immune destruction of the glands. It might also develop if the glands were inadvertently removed when the thyroid glands were removed. Lack of the hormone lowers calcium levels and signs include seizures, muscle twitching and tremors, trouble walking, and weakness.
- Hyperparathyroidism: This condition is caused by the oversecretion of the parathyroid hormone. It may arise with either benign of cancerous tumors of the gland and mostly seen in older cats. Calcium levels become extremely high, resulting in kidney damage. Symptoms include increased urination, vomiting, loss of appetite, lethargy, and muscle weakness.
- Diabetes mellitus: Also known as sugar diabetes is a disease in the pancreas. This common disorder arises from the underproduction or inappropriately low secretion of insulin. This 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.
- Cushing’s disease: Also known as hyperadrenocorticism, this common disease involves the overproduction of cortisol. It is usually seen in middle aged to older cats as a secondary to an overproduction of the hormone ACTH by the pituitary gland. Affected cats may have very nonspecific signs such as lethargy, decreased appetite, weight loss, dehydration, weakness, and vomiting.
- Addison’s disease: A less common disease of the adrenal gland is hypoadrenocorticism. It is caused by a deficiency of two hormones: cortisone and aldosterone. Affected cats are often young and have nonspecific clinical signs such as weakness, lethargy, vomiting, diarrhea, and blood in the bowel movements.
- Pheochromocytoma: A tumor in the adrenal gland is an extremely rare cause of high blood pressure in a cat. This tumor causes the overproduction of norepinephrine hormone in cats. It also occurs primarily in older cats.