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.