The endocrine system of dogs and cats is almost identical to that of humans, in structure and function. The endocrine system is made up of a number of glands in the body that secrete hormones to control a range of important functions – from growth and metabolism, to reproduction, blood pressure, mineral balance and homeostasis in the body. Its overall purpose is the regulation and control of various functions to keep the body in balance. The canine or feline body is in a constant state of detecting stimuli (threats, thirst, reproductive opportunity, hunger, temperature, etc.). It is the endocrine system that responds to those external stimuli in ways that keep the body in balance.
In this article, we’ll explore the structure and organs of the canine and feline endocrine system, their functions, how the endocrine system works and what happens when something goes awry with this system. We’ll also discuss what to do if you think your pet has a problem with their endocrine system.
Organs of the endocrine system
The endocrine system is made up of a variety of glands that secrete hormones (or ‘chemical messengers’) that control various functions in the body. The glands of the endocrine system are ductless – meaning they secrete hormones into tiny capillaries that then carry the hormones to the targeted organs via the bloodstream.
Using the structure of the endocrine system, we’ll briefly explain how each gland works, the hormone/s it secretes and the effects it/they have on the body.
Pituitary gland
Location
The pituitary gland is a very small structure located at the base of the brain.
Hormones secreted
• Growth hormone (GH) – regulates growth in the body
• Melanocyte-stimulating hormone (MSH)
• Prolactin and oxytocin – stimulate the production of milk and lactation after birth
• Antidiuretic hormone (ADH) or vasopressin – controls the balance of water in the body, through kidney function
• Adrenocorticotropic hormone (ACTH) – stimulates the adrenal glands
• Thyroid-stimulating hormone (TSH) – stimulates the thyroid gland
• Follicle-stimulating hormone (FSH) and luteinising hormone (LH) – stimulate the gonads (ovaries and testes)
Function
Some of the hormones secreted by the pituitary gland influence processes in the body directly, while some of the hormones stimulate other glands in the endocrine system. In these instances, the pituitary gland functions as a vital link between the nervous system and the rest of the endocrine system. It is sometimes referred to as the ‘master gland’.
Thyroid gland
Location
The thyroid gland consists of two lobes located on either side of the trachea.
Hormones secreted
• triiodothyronine (T3)
• thyroxine (T4)
Function
The thyroid gland has an influence over every cell in the body as it regulates functions like metabolism, growth, heartrate, body temperature, digestion, the maintenance of skin and bone, and brain development. The pituitary gland stimulates the thyroid with thyroid-stimulating hormone (TSH) and the thyroid secretes T3 and T4 into the bloodstream to help regulate the body’s various functions and states.
Parathyroid glands
Location
The parathyroid glands are also located in the neck of the dog or cat, behind the lobes of the thyroid gland. There are four parathyroid glands – two on each side of the thyroid.
Hormones secreted
• parathyroid hormone (PTH), which controls calcium metabolism
Function
The parathyroid glands’ only function is to secrete PTH, which targets the kidneys, intestines and bones, and regulates blood calcium. When calcium levels drop too low, it can have a negative health effect, so the parathyroid gland reacts fast to restore calcium levels and keep the body healthy.
Pancreas
Location
The pancreas is located near the liver and stomach, in the upper abdomen of dogs and cats.
Hormones secreted
• insulin
• glucagon
Function
The endocrine pancreas secretes insulin and glucagon to keep the body’s blood sugar levels stable. These hormones ensure that glucose and other necessary energy compounds are transported to all of the body’s cells.
Adrenal glands
Location
The adrenal glands are located in the abdominal cavity, just in front of the kidneys.
Hormones secreted
• mineralocorticoids, which control blood pressure by balancing sodium, potassium and water
• glucocorticoids (cortisol), which regulate the metabolism, control blood sugar, harness protein and fat for energy, and assist the immune system in fighting fever and illness
• sex hormones (testosterone, oestrogen, progesterone)
• epinephrine or adrenaline, which assists the body to respond to stressful stimuli by opening the airways, dilating blood vessels to the muscles, and increasing the heartrate
• norepinephrine balances normal heartrate, blood pressure and breathing
Function
The adrenal glands (along with the hypothalamus and pituitary gland) help to control the body’s reaction to a threat (or stress) as well as restoring its equilibrium. The secreted hormones also play important roles in blood sugar control, digestion, metabolism, immunity, water and electrolyte balance, reproduction, and blood pressure.
Ovaries and testes
Location
Canine and feline gonads (testes in males and ovaries in females) are located near their genitals. In males, the testicles are outside of the abdomen in the scrotum, below the anus (in cats) or between the thighs (in dogs). In females, the ovaries are located near the top of each uterine horn (left and right) in the abdomen.
Hormones secreted
• testosterone is produced by the testes
• oestrogen and progesterone are produced by the ovaries
Function
The hormones secreted by the testes and ovaries help the dog or cat to develop distinct male or female characteristics, respectively. Male and female hormones prepare the animals for sexual reproduction, and assist the foetuses to develop and grow.
Endocrine-related health issues in pets
As you can see, each endocrine gland secretes either one or several hormones that target other glands or organs to keep the dog or cat’s body in balance. However, when either too much (hyper-) or too little (hypo-) of a hormone is secreted, it can cause an imbalance in the very finely-tuned endocrine system and cause health issues. We’ll look at some of the more common examples:
Pituitary gland
A pituitary tumour, an injury to- or infection of the pituitary gland can affect its ability to produce the right amount of hormones targeting other endocrine glands or development in the body. The following endocrine disorders are caused by pituitary tumours or disease.
Acromegaly
Caused by a slow-growing, usually benign pituitary tumour, acromegaly is the physical result of the excess secretion of growth hormone. It is more common in cats and very rare in dogs. Symptoms include abnormal growth of the soft tissues of the face and skull, enlargement of the paws and legs, as well as the progressive enlargement of internal organs like the heart, liver and kidneys. Acromegaly triggers early-onset diabetes mellitus, so the cat will show diabetes symptoms such as excessive thirst, urination, and weight gain.
Short-term medication of acromegaly symptoms can provide some relief, but the long-term treatment involves reducing or destroying the pituitary tumour with radiation therapy.
Hyperadrenocorticism (Cushing disease)
Even though Cushing disease is caused by excess cortisol in the system – a result of overstimulation of the adrenal glands – it’s considered a pituitary disease because it is triggered by a small tumour on the pituitary gland. There are, however, a few cases where Cushing disease is caused by a tumour on one of the adrenal glands.
The symptoms of Cushing disease include excess thirst and urination, panting, overheating, fatigue, hair loss and thin skin, easy bruising, increased appetite (which leads to obesity) and a potbellied appearance. A conclusive diagnosis of Cushing disease is often difficult because of other diseases that may be present. If the vet suspects Cushing disease, they will need to test and then retest at a later stage using specialised blood and urine tests. Once confirmed, the vet will then test to see whether the disease is caused by a tumour on the pituitary gland or the adrenal gland/s, as this will determine which treatment to provide.
Panhypopituitarism
This condition occurs when the pituitary gland is compressed due to a tumour, infection or injury. When this happens in adulthood (adult-onset panhypopituitarism), it restricts the pituitary gland’s ability to secrete hormones, which impedes the hormone secretion of all the other endocrine glands. This results in a wide range of severe symptoms such as a lack of co-ordination, collapse, weight loss, indifference, and hiding. Excessive thirst and urination are common. A tumour near the pituitary gland can also compress the optic nerve and result in blindness.
Juvenile-onset panhypopituitarism is also called pituitary dwarfism because part of the pituitary gland in puppies (and very rarely in kittens) does not develop or is compressed by a tumour. This results in stunted growth, which includes developmental problems like delayed dentition, absence of guard hair, loss of coat, small genitalia, and other symptoms of underproduction of endocrine hormones. The prognosis for pituitary dwarfism is poor – the condition shortens the pup’s lifespan considerably.
Diabetes insipidus
Unrelated to blood sugar levels and insulin, diabetes insipidus occurs when the pituitary gland cannot secrete enough antidiuretic hormone (vasopressin) or the kidneys respond abnormally to it. The condition results in problems with fluid retention in the body. The animal will urinate in large volumes and drink excessive amounts of water.
When diagnosing diabetes insipidus, the vet will rule out kidney disease and dehydration before running carefully-monitored water deprivation tests. They may treat the dog with an antidiuretic hormone to see what the response is.
Thyroid gland
The thyroid hormones control the metabolism, which means they affect almost every part of the body. Therefore, when too much or too little of the thyroid hormones T3 and T4 are secreted, it results in a range of severe symptoms.
Hypothyroidism
The under-secretion of thyroid hormones may be due to a pituitary tumour or a problem with the thyroid gland itself. Dogs sometimes have a congenital thyroid problem at birth, but hypothyroidism in dogs usually develops between the ages of four and 10 years old. It is very rare in cats.
Because it’s mainly the metabolism that is affected, the symptoms of hypothyroidism can include dullness, fatigue, unwillingness to exercise, weight gain (with no increase in appetite), dry skin and brittle coat (with hair loss and thinning), thickened skin, and nerve paralysis. The vet will perform tests to confirm the diagnosis, and treatment involves giving synthetic thyroid hormones to compensate for the low thyroid function.
Hyperthyroidism
A thyroid gland tumour causes the thyroid to overproduce T3 and T4 hormones, causing hyperthyroidism. This unnatural increase in the metabolism can produce symptoms such as increased heartrate, difficulty breathing, weight loss, excessive thirst and an increase in appetite. Vomiting and diarrhoea can also present. It is far more common in cats than in dogs.
Hyperthyroidism is most often treated with radioactive iodine to destroy the thyroid tumour.
Parathyroid glands
Since the parathyroid glands are so close to the thyroid glands, they may be affected by any condition, damage to, infection, or removal of the thyroid glands. The parathyroid gland’s regulation of calcium in the body is important for maintaining the health of the skeleton, muscles, blood, nervous system and hormones in general. A disruption in this calcium regulation can have far-reaching effects on the animal.
Hypoparathyroidism
The underproduction of parathyroid hormone causes hypocalcaemia – abnormally low blood calcium levels. The symptoms can include muscle twitching, spasms and seizures. This can occur when the thyroid is removed and the parathyroid glands are also accidentally removed, or damaged during surgery.
This is uncommon in dogs, but common in cats who have had thyroid surgery or previous thyroid issues. Treatment includes lifelong supplementing with calcium and vitamin D.
Hyperparathyroidism
The overproduction of parathyroid hormone is very rare in dogs and cats, and may cause hypercalcaemia – abnormally high levels of blood calcium. This is destructive to the internal organs – most notably the heart and kidneys – as well as the nervous system and blood vessels. It can become life-threatening without rapid treatment.
Symptoms include tremors and spasms, pain in the jaw, cramps, fever, weakness, and restlessness.
Pancreas
Insulin, produced in the pancreas, influences every single cell in the body because it’s responsible for balancing blood sugar and putting glucose to use in the body. The pancreas also makes glucagon, which converts carbohydrates to energy when blood glucose is not available. Together, insulin and glucagon keep blood glucose in balance, and the body energised.
Diabetes mellitus
When the body becomes resistant to insulin or the pancreas cannot produce enough insulin, this causes diabetes mellitus. In dogs, diabetes is most often found in middle-aged small breed females. It’s also a symptom of Cushing disease, and is common in dogs and cats that are obese. In cats, middle-aged, obese, neutered male cats are more likely to develop diabetes.
The symptoms of diabetes include excessive thirst and urination, weight loss even with an increase in appetite, and recurring infection without healing. Vision problems like cataracts can occur, and the liver is enlarged. Treatment for diabetes mellitus includes insulin injections (or oral medication) as well as frequent check-ups and adjustments to treatment as the condition improves.
Functional islet cell tumours (insulinomas)
An insulinoma is a tumour that produces large amounts of insulin, which decreases blood sugar, causing hypoglycaemia, even if the dog or cat is eating normally. Early symptoms may not be obvious, but become more pronounced as the disease progresses. The signs of hypoglycaemia include weakness and fatigue, muscle twitching, incoordination, changes in temperament and even seizures. Symptoms improve with glucose treatment, but chronic hypoglycaemia can cause brain damage.
Insulinomas are usually malignant, and more aggressive in dogs than in cats.
Adrenal glands
The adrenal glands are the target glands for adrenocorticotropic hormone secreted by the pituitary gland. Over- or under-production of adrenal hormones can have a pituitary cause or be caused by a problem with the adrenal glands.
Hyperadrenocorticism (Cushing disease)
(See pituitary illnesses above)
Hypoadrenocorticism (Addison’s disease)
Very rare in cats, Addison’s disease occurs more frequently in dogs, and is caused by the under-production of mineralocorticoids and glucocorticoids. This under-production is usually due to a failure of the adrenal gland because of immune-mediated gland destruction (the immune system attacking the dog’s own glands) or a metastatic tumour.
The symptoms of Addison’s disease usually start with gastrointestinal issues like vomiting and diarrhoea, loss of appetite, weight loss, increased urination and thirst, and weakness. The dog loses body condition and then rapidly loses weight and is at risk of renal failure. The vet will perform multiple tests, but diagnosis is made by evaluating adrenal function and the amount of circulating cortisol.
Phaeochromocytomas
Sometimes tumours on the adrenal medulla show no symptoms and are discovered by accident on diagnostic imaging for another condition. The tumours themselves secrete adrenal hormones, and when symptoms start to show, they include excessive thirst and urination, distended abdomen, high heartrate, high blood pressure, and restlessness.
When should you see the vet about your pet’s endocrine system?
The symptoms of these endocrine disorders may resemble so many other illnesses or health issues – especially those that are gastrointestinal or neurological in nature. If your pet is showing abnormal behaviour or appearance; if they are losing fur or doing anything that seems out of the ordinary, make a note of their symptoms and make an appointment with the vet as soon as possible. If they are panting or trembling or collapsing, try to take a video to show the vet, which will give the vet a better indication of what’s going on in real-time.
If anything is happening to your pet that affects their eating, sleeping, elimination or general activity in any way, your best course of action is a vet visit.
How do you take care of your pet’s endocrine system?
Unless your pet was born with an obvious endocrine disorder, it’s difficult to predict whether they will develop any conditions relating to the under- or over-secretion of endocrine hormones.
Aside from maintaining a daily routine with your pet in which you feed, exercise, groom and keep them stimulated at more or less the same time every day, it’s also important to make sure they do not gain excess weight. Obesity can trigger and/or exacerbate endocrine issues like diabetes mellitus, hypothyroidism, Cushing disease, insulinoma, and a deficiency in growth hormone.
If your pet is showing abnormal behaviour within their daily routine, it will be easier to spot it than if their day was haphazard.
Be sure to feed your pet a healthy diet, give them regular exercise and take them to the vet for preventative treatment and an annual check-up. This will give the vet the opportunity to thoroughly go over your pet’s health and to spot anything out of the ordinary. Always contact the vet immediately when your dog or cat looks ill or if their habits change.
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