A Quick Tour of the Endocrine System

Think of the endocrine system as your pet’s hormone communication network. It’s made up of glands that produce hormones, chemical messengers that help different parts of the body work together.

And these glands don’t work alone. They constantly communicate with the immune system, gut, brain, and nervous system to help keep the body in balance.

Here are some of the key players:

• Hypothalamus & pituitary gland: Located in the brain, these act like the body’s command center, sending signals that help coordinate hormone production throughout the body.
• Thyroid: Located in the neck, the thyroid helps regulate metabolism, energy, growth, and many other body functions.
• Adrenal glands: These small glands sit above the kidneys and help the body respond to stress. They also produce hormones involved in metabolism, blood pressure, and immune regulation.
• Ovaries & testes: These reproductive organs produce hormones such as estrogen, progesterone, and testosterone. These hormones do more than support reproduction. They also influence immune function, bone and joint health, and metabolism.
• Pancreas: Best known for its role in digestion, the pancreas also produces insulin, which helps regulate blood sugar. When inflammation and other factors interfere with insulin signaling, blood sugar regulation can become more difficult.
• Parathyroid glands: These tiny glands sit near the thyroid and help maintain the body's calcium balance. Calcium is important for healthy muscles, nerves, bones, and communication between cells, including immune cells.

The big picture: Your pet’s hormones are part of a much larger conversation happening throughout the body. When one part of this system changes, it can influence other systems too, which is why hormonal health deserves a place in the bigger picture of immune health.

The Thyroid-Immune Connection

The thyroid deserves its own section. It's one of the most common hormone issues we see in dogs, less often in cats, and its ties to immune function run deep. An underactive thyroid, or hypothyroidism, slows nearly everything: metabolism, energy, coat quality, and immune regulation along with it. Autoimmune thyroiditis adds another layer to watch for, a condition where the immune system's own antibodies target the thyroid gland, which means the thyroid can be both a casualty of immune dysfunction and a contributor to it.

A sluggish thyroid doesn't just mean a tired pet. It can mean an immune system that isn't getting the metabolic support it needs to do its job well.

A Note on Cats and Hyperthyroidism

Everything above describes the pattern we see most often in dogs. Cats tend to run the opposite direction. Feline hyperthyroidism, an overactive thyroid usually caused by a benign growth on the gland, is one of the most common hormone disorders in older cats, and it speeds nearly everything up instead of slowing it down. Weight loss despite a hearty appetite, restlessness, a racing heart rate, and a rough or greasy coat are the signs I look for first.

Because an overactive thyroid also revs up metabolic demand on the immune system, untreated hyperthyroid cats can look deceptively bright and energetic while the underlying strain builds. A senior cat losing weight with a good appetite is one of the clearest signals in veterinary medicine that bloodwork, not a wait-and-see approach, is the right next step.

The HPA Axis and Chronic Stress

The HPA axis, short for hypothalamic-pituitary-adrenal axis, is the pathway your pet's body uses to respond to stress. In a well-regulated system, it works like a thermostat. The brain senses a stressor, signals the adrenal glands to release cortisol, cortisol does its job, and the system dials back down once the stress has passed.

Chronic stress throws off that rhythm. When stress doesn't let up, whether it's coming from pain, an unsettled diet, gut inflammation, or an environment that never quite feels safe, the feedback loop stops working cleanly. Cortisol production becomes less predictable, sometimes running too high, sometimes too low. A 2024 review in the journal Animals found that persistent HPA axis activation and prolonged cortisol release can impair canine immune function, which is a meaningful part of why we treat minimizing stress as an immune-health strategy in its own right, not simply a comfort issue.^[1]

This doesn't mean every stressed pet develops disease, and it doesn't mean every health issue traces back to stress. It means stress is a real, physiological input into how well the immune system functions, alongside diet, gut health, and hormones.

A Careful Look at Spay, Neuter, and Hormones

This is a topic I want to handle carefully. Spay and neuter surgery remains one of the most important tools we have for preventing unwanted litters and certain reproductive diseases, and nothing here argues against the procedure. This is about understanding one more piece of your pet's hormonal picture so you can have a fuller conversation with your veterinarian.

Spaying and neutering remove the ovaries or testes, which permanently changes the body's hormone feedback loop. Under normal circumstances, the pituitary gland releases luteinizing hormone, or LH, which signals the gonads to produce sex hormones. Those sex hormones signal back to the pituitary to slow LH production, a classic feedback loop. Once the gonads are removed, that feedback signal disappears, and LH levels can rise well above normal and stay elevated for the rest of the pet's life.

Dr. Michelle Kutzler at Oregon State University has led research finding that LH receptors exist in tissues well beyond the reproductive system, including the thyroid, the gastrointestinal tract, and certain joint ligaments. Her 2020 review in the journal Animals proposes that ongoing LH receptor activation in these tissues may be one contributing factor in some of the health effects seen more often in spayed and neutered dogs, though the mechanisms are still being studied and aren't fully settled.^[2]

Separately, breed-specific research out of UC Davis, led by Dr. Benjamin Hart, has examined the timing of spay and neuter surgery against certain joint disorders and cancers in specific breeds, most notably Golden Retrievers. The 2013 study, published in PLOS ONE, found that the age at which a Golden Retriever was neutered was associated with different rates of certain joint conditions and cancers.^[3] Later research expanding to 35 breeds found these associations vary significantly by breed and by sex.^[4] This remains an active, evolving area of research, and veterinarians don't all agree on how to weigh it against the well-established benefits of spay and neuter.

Supporting the Adrenal Glands

If the HPA axis is the feedback loop, the adrenal glands are where a lot of the actual work happens. These small glands, sitting on top of the kidneys, produce cortisol along with hormones that help regulate blood pressure and fluid balance.

When a pet is under chronic stress, whether from pain, an unsettled home environment, or an ongoing health issue, the adrenal glands can end up working overtime for extended stretches. Supporting them isn't complicated. It comes down to reducing unnecessary stressors where you can, keeping a predictable routine, and making sure your pet's diet includes adequate B vitamins, which the body relies on more heavily during periods of stress. Some integrative veterinarians also use adaptogenic herbs for individual pets, but these should be chosen and dosed with your vet's guidance, not picked off a shelf.

Sleep matters here more than most pet parents realize. A dog or cat who doesn't get uninterrupted rest, because of noise, a shared sleeping space with tension, or a schedule that keeps shifting, often shows the same signs as one under chronic stress: irritability, digestive upset, a coat that never quite looks right. A quiet, consistent sleep space is one of the simplest ways to give the adrenal glands room to reset overnight.

Signs Your Pet's Hormones May Need Support

Weight changes without a change in diet, a dull or thinning coat, low energy, and mood shifts can all point toward a hormone imbalance. These same signs can point to plenty of other things too, which is exactly why testing matters more than guessing.

• Weight change without a diet change. Gradual gain or loss that doesn't match feeding or activity levels.
• Coat changes. Dull, thinning, greasy, or slow-to-regrow fur.
• Energy shifts. Noticeably more tired, or in cats, unusually restless.
• Mood changes. New irritability, anxiety, or withdrawal without an obvious cause.

Nutrition as a Hormone-Supportive Foundation

Balanced, whole-food nutrition gives the endocrine system the raw materials it needs to function well. The thyroid depends on adequate iodine, and selenium plays a role in converting thyroid hormone into its active form. A whole-food diet that includes a reliable iodine source, like kelp in appropriate amounts, supports this system far better than a diet that leaves these nutrients to guesswork.

Protein quality plays a role too. Hormone production depends on amino acids, the building blocks protein breaks down into, and a diet built around identifiable, whole-food protein sources gives the endocrine system more to work with than a diet where the protein source is a vague, processed meal. Zinc and vitamin A, both found in organ meats in appropriate amounts, also support thyroid hormone conversion and immune signaling.

Working Alongside Your Veterinarian

Hormone health is genuinely complex, more so than most of the other pieces we've covered this month, and your veterinarian is the right partner for any testing, medication, or surgical decisions. Nothing here replaces that relationship. My goal is to help you ask better questions and bring more useful information to the conversation you're already having with them.

Of the four pillars in this series, hormones are probably the one pet parents think about least, mostly because the connections aren't obvious day to day. A pet's coat, energy, weight, and mood can drift slowly enough that changes are easy to miss until they've been building for months. That's exactly why the simple habit of paying attention, and bringing what you notice to your vet, matters here more than almost anywhere else in this series.

Frequently Asked Questions

Can hormone imbalances affect my dog's immune system?

Yes. The endocrine and immune systems share signaling pathways, so a hormone imbalance, especially involving the thyroid or adrenal glands, can change how well the immune system regulates inflammation and responds to challenges. This is why an endocrine workup is often worth considering alongside gut and inflammation testing when a pet has ongoing, low-grade health issues.

What are the signs of thyroid problems in dogs?

Weight gain without a change in diet, low energy, a dull or thinning coat, and cold intolerance are the most common signs of hypothyroidism in dogs. Because these signs overlap with several other conditions, a full thyroid panel, not just a basic screening test, gives your veterinarian the clearest picture.

Does spaying or neutering affect my pet's hormone levels?

Yes. Removing the ovaries or testes eliminates the negative feedback loop that normally regulates luteinizing hormone, so LH levels rise and stay elevated for life. Researchers are still studying what that means long-term, and it's a good topic to raise with your vet rather than a reason to avoid the procedure.

How do I know if my pet needs a full thyroid panel?

If your pet shows unexplained weight gain, low energy, coat changes, or a combination of small symptoms across different systems, ask your veterinarian for a full panel that includes free T4 and thyroid antibody testing, not just a basic total T4 screen.

Is hyperthyroidism in cats related to their immune system too?

Yes, indirectly. An overactive thyroid speeds up metabolic demand throughout the body, including on the immune system, which is part of why untreated hyperthyroid cats can eventually show secondary issues like heart strain or muscle wasting. Early bloodwork in senior cats catches this well before it reaches that point.