Equine Health and the Endocrine System

The endocrine system is one of the major regulatory systems in the horse's body, along with the nervous system. It is responsible for producing, releasing, and regulating hormones, which are the chemical messengers that control various body functions. This includes controlling the horse's metabolism, growth and development, reproduction, and response to stress. 

“At any given time, 30 to 40 different hormones are in distribution in your horse’s bloodstream, orchestrating energy usage, water and mineral balance, heart rate and blood pressure, growth, and sexual development, reproduction and stress reactions.” – Equus Magazine

In horses, as in humans, there are several key glands that make up the endocrine system. These include the pituitary gland, thyroid gland, adrenal gland, pancreas, and gonads (testes or ovaries). Each gland produces specific hormones that have different functions in the body. Changes in the function of the endocrine system can lead to significant hormone imbalances and clinical dysfunction, including Pituitary pars intermedia dysfunction (PPID, aka Cushing’s disease) and Equine Metabolic Syndrome (EMS). 

In this blog, we’ll cover these key glands, what they do, and the equine health issues that may result in each area. 

The Pituitary, or “Master” Gland

The pituitary gland is sometimes called the "master gland" because it produces several hormones that control other glands in the body. For example, it produces growth hormone, which stimulates the growth and development of bones, muscles, and other tissues. It also produces adrenocorticotropic hormone (ACTH), which stimulates the adrenal gland to produce cortisol, a hormone that helps the horse cope with stress.

Affects Circulation and Kidney Function

The hypothalamus produces an anti-diuretic hormone which is released by the pituitary. This hormone acts in response to hypotension (low blood pressure), increasing blood pressure by signaling the kidneys to retain water and constricting small arteries when blood pressure drops.

Affects Metabolism and Growth

The pituitary gland secretes a thyroid-stimulating hormone, which triggers hormone synthesis and secretion in the thyroid gland. It also produces adrenocorticotropic hormone (ACTH) to regulate steroid production in the adrenal glands. You may be familiar with ACTH as a Cushing’s/PPID marker. The growth hormone produced by the pituitary gland orchestrates protein synthesis, carbohydrate metabolism, and related processes throughout the body.

Affects Reproduction

In horses and humans, the reproductive cycle is influenced by pituitary hormones. These hormones include:

• Follicle stimulating hormone and luteinizing hormone affect the ovaries by stimulating the development and maturation of the follicle and the testes by aiding in sperm maturation. This affects conception and pregnancy.

• Luteinizing hormone affects the production of testosterone and estrogen.

• Prolactin supports milk production

• Oxytocin is produced by the hypothalamus, which triggers smooth contractions during birth, the expulsion of the placenta, and milk ‘let down.’

Equine Health Issues Related to the Pituitary Gland

In horses with Cushing's disease, the pituitary gland releases too much of the hormone ACTH. This hormone tells the adrenal glands, which are located near the kidneys, to produce more cortisol, a hormone that helps the body respond to stress.

Normally, the pituitary gland would stop releasing ACTH when there is enough cortisol in the body, but in horses with Cushing's disease, this feedback mechanism doesn't work properly. As a result, the adrenal glands continue to produce too much cortisol, which can cause a variety of symptoms, such as a long, curly coat, increased thirst and urination, and a pot-bellied appearance.

The Thyroid Gland

Hormones produced by the thyroid gland (mainly thyroxine and triiodothyronine) regulate metabolism, which is the process by which the body converts food into energy. Thyroid hormones also affect protein synthesis and breakdown, which sets the basal metabolic rate, helps regulate the rate at which the body burns calories and produces heat, and is important for maintaining body temperature in horses.

Thyroid hormones in young horses promote growth and organ maturation. The thyroid gland regulates calcitonin, which is involved with calcium homeostasis between bone and circulation. 

Parathyroid glands 

There are four parathyroid glands that produce the parathyroid hormone, which regulates calcium levels in the body. This hormone can: 

• Stimulate bones to release calcium into the bloodstream 

• Cause kidneys to retain calcium rather than eliminate it in urine

• Trigger the gastrointestinal tract to take more calcium from food

• Release phosphorous into the urine

Equine Health Issues Related to the Thyroid/Parathyroid Glands

The thyroid gland plays an important role in weight management. Because the hormone thyroxine determines the metabolic rate, a horse with low thyroxin levels will be an easier keeper. 

The thyroid and parathyroid glands also try to maintain a proper calcium-to-phosphorus ratio in your horse’s system. Too little calcium relative to the amount of phosphorus can result in weakened bones, bone fractures, and lameness. Excessive calcium levels can lead to mineral imbalances, which can cause soft tissue calcification, kidney damage, and urinary tract problems.

The Adrenal Glands

Located on the ends of the kidneys, the adrenal glands produce several hormones that affect the horse’s metabolism, behavior, and stress. One of these hormones is adrenaline (also known as epinephrine), which increases heart rate and blood pressure and prepares the body for "fight or flight" responses. The adrenal gland also produces cortisol, which helps the horse cope with stress over a longer period of time.

“The kidney also partially acts as an endocrine organ in this pathway by producing renin, which, through the renin-angiotensin system, stimulates aldosterone production by the adrenals.” EquiMed

Glucocorticoids and Metabolism

Adrenal glands produce glucocorticoids (cortisol, cortisone, corticosterone) which regulate energy, protein, fat and mineral metabolism.  Glucocorticoids help to reduce inflammation and, during periods of persistent stress, alter the horse’s metabolism and body functions.

Mineralocorticoids and Circulation

Adrenal glands also produce mineralocorticoids, including aldosterone, which regulates sodium levels, water balance, and blood pressure.

Gonadocorticoids and Reproduction

Gonadocorticoids, produced by the adrenal glands, act on sperm production and affect reproductive and gender-specific behavior in both sexes.

Catecholamines and Stress

Finally, the adrenal glands produce catecholamines (epinephrine, norepinephrine, and dopamine) which determine the fight or flight response to perceived threats.

Equine Health Issues Related to the Adrenal Gland

Poor immune health due to a lack of glucocorticoids may be signaled by persistent dermatitis, abscesses, and slow healing. Additionally, overproduction of cortisol is a contributing factor in equine Cushing’s Disease.

The Pancreas

The pancreas produces insulin, which regulates the horse's blood sugar levels. Insulin helps to move glucose (a type of sugar) from the bloodstream into the cells, where it can be used for energy. Insulin is also the primary regulator of blood sugar levels. The pancreas also produces glucagon, which functions to pull glucose out of storage when blood sugar levels are low.  

Enzymatic juices produced by the pancreas are secreted into the small intestine to digest feed.

Equine Health Issues Related to the Pancreas

If insulin levels are too high or too low, it can lead to metabolic dysfunction, including insulin resistance (IR).

The Gonads

The gonads (testes or ovaries) produce hormones involved in reproduction, such as testosterone and estrogen. These hormones play a role in the development of secondary sex characteristics and the regulation of the reproductive cycle in mares and stallions.

More About Estrogen

We often see estrogen and testosterone imbalances reflected in the horse’s behavior because, in mares, high levels of estrogen can cause a condition called "estrous behavior," which is when a mare is in heat and shows signs of being ready to mate. This behavior can make a mare difficult to handle, even exhibiting stallion-like behavior such as aggressiveness or mounting other horses. 

Improper estrogen levels carry health risks too. If the mare’s endocrine system can’t regulate estrogen production on its own, she may have irregular or prolonged estrous cycles, which can make breeding difficult or impossible. It can interfere with her ability to conceive or carry a pregnancy to term and can lead to reproductive disorders, including ovarian cysts, uterine infections, and infertility. 

Testosterone

In stallions (male horses), high levels of testosterone can cause aggressive behavior and make them difficult to handle. It can also affect their fertility if their testosterone levels are too low. 

Testosterone is also involved in muscle development and maintenance, and imbalances can affect a stallion's muscle mass and strength. Low testosterone levels can result in muscle wasting or weakness, while high levels can lead to increased muscle mass but may also cause negative side effects such as lameness or joint stress.

Conclusion

It’s easy to see how vital the endocrine system is to your horse’s body functions and overall health. The good news is that endocrine disease in horses is not common. And, having some knowledge of the organs that guide our horse’s development and behavior gives us a little more insight into who they are and what’s making them tick. . 

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