Skin and Coat Clues to Systemic Disease: 5 Warning Signs
Written by Dr. Sarah Calvin, DVM
Dr. Sarah Calvin, DVM is a small-animal veterinarian with interests in dermatology and internal medicine. She supports La Petite Labs by helping ensure our skin, coat, and nail education remains biologically accurate, conservative, and easy for pet parents to understand.
The integumentary system is the largest organ system and is comprised of skin, hair, nails, and associated glands. It is also what we see most when we look at our pets. Let this be a window to your pet’s health. In veterinary medicine, it is referred to as a “diagnostic surface”: a visible system that reflects multiple internal systems. When pet owners know what changes to look for, they can often spot warning signs early – sometimes before more serious symptoms appear.
Animal bodies are incredibly adaptive. When an internal disease develops, internal signals shift the body from growth and maintenance to survival and defense. Hair and nails are often the first tissues affected. Their growth is metabolically expensive, requiring energy and proteins, and their function is not essential for immediate survival. By slowing cellular processes in hair and nails, the body can redirect resources and energy to more critical systems.
The skin and coat are also tightly regulated by the endocrine (hormonal) and immune systems. When either of these systems become dysfunctional, changes will become evident in the skin and coat.
Let this knowledge be a tool for awareness, not anxiety. Understanding these five warning signs can help you become an informed and proactive pet owner.
“Understanding these five warning signs can help you become an informed and proactive pet owner.”
1. Hair Loss
Hair loss in pets can range from subtle increases in shedding to shocking bald patches. There are many possible causes. An important first question is whether your pet is itchy.
Hair loss caused by infections, allergies, or parasites are usually accompanied by itchiness. In contrast, hair loss without itching may be a sign of a metabolic disorder or stress. The two most common metabolic causes are hypothyroidism and hyperadrenocorticism (Cushing’s disease).
Thyroid hormone functions like the body’s metabolic gas pedal. Hypothyroidism (low thyroid levels) is like taking your foot off the gas: all metabolic processes slow down. This condition occurs in dogs and cats, and often results in lethargy, weight gain, and coat changes. The coat often becomes thin and grows poorly when trimmed. Thyroid hormone is essential for keratin and protein synthesis within the hair follicle, as well as maintaining adequate blood flow to the skin. Without sufficient thyroid hormone, hair cannot properly enter and maintain the growth phase of the hair cycle.
Cushing’s disease also causes hair loss, but through a different mechanism. It results from excess cortisol in the body, either produced by the body or administered as a medication. Cortisol is a catabolic hormone, meaning it breaks down tissues. This suppresses the follicle (causing hair to stop growing) and causes the skin to atrophy (resulting in fragile skin and poor growing conditions).
A third major cause for hair loss is overgrooming, which can occur with or without itch. Animals self-soothe through repetitive movements like grooming. When anxious, bored, or stressed, repetitive grooming can help calm the nervous system. Hair loss in these cases often appears as distinct bald patches, commonly on the legs, sides, or abdomen.
2. Recurrent Infections
Your pet’s skin is home to trillions of bacteria, fungi, and viruses. Under normal conditions, the skin acts as a barrier, preventing these organisms from causing disease. If a breach occurs, the immune system works alongside the skin to control microbial growth.
When the immune system becomes compromised, once tolerated organisms can become a problem. In many pets, the first signs of a suppressed immune system appear on the skin, often as skin infections – red, itchy, or malodorous regions. You may see circular, scaly lesions known as “epidermal collarettes.”
The most common cause of recurrent skin infections in dogs and cats is allergies – an overreaction of the immune system. However, similar symptoms can be a result of an underactive or suppressed immune system, making it unable to control normal skin microbes.
3. Poor Grooming (Cats)
Our pets are naturally wired to maintain their coat and nails. This section will focus on cats specifically since they are especially meticulous groomers. In fact, a noticeable decline in grooming almost always signals an underlying problem in cats. Signs your cat may be grooming less include: matting hair, clumps of hair that are easy to remove, increased dander, and increased hair removed during brushing sessions.
Cats need to contort their bodies to groom every surface. There are three main factors that can interfere with this ability: pain, poor mobility, and lethargy.
Pain is the most common cause of decreased grooming in cats. Arthritic joints, dental disease, and muscle soreness can all make grooming uncomfortable or impossible.
A veterinarian can help identify sources of chronic pain and develop a management plan. Be sure to mention if your cat experiences any of the following:
- Decreased time on elevated surfaces (a sign of pain while jumping)
- Stair avoidance or taking steps one at a time
- Grooming mostly focused on legs and paws
- Urinating and/or defecating outside of the litter box
- Changes in behavior like hiding, aggression, and decreased social time
- Decreased appetite, eating slowly, or dropping food
Poor mobility may prevent cats from reaching certain areas, particularly on the back and base of tail. Obesity is the most common cause, followed by joint stiffness. Cats should have a visible waist when viewed from above. With light pressure, you should be able to feel their ribs, spine, and hip bones.
Finally, lethargy could be the cause of decreased grooming. Cats spend an average of 3-4 hours grooming each day. Lethargy can significantly decrease this time, resulting in a poorly kept coat. Lethargy has many potential causes and should always prompt a veterinary evaluation.
4. Dandruff
Dandruff in animals – formally called seborrhea – results from a defect in keratinization: the process by which skin cells mature and form the outer layer of protective skin. This outer layer contains a strong, structural protein called keratin.
Hormones are a major regulator of keratinization. When hormone imbalance occurs, keratin production and shedding become disordered, leading to flaky, scaly skin.
Hypothyroidism is a classic example. As metabolic pathways slow, skin cell turnover decreases. New cells do not form properly, and old cells shed abnormally. As a result, dead skin cells build up, flake, and cause dandruff.
Other causes of dandruff include allergies, improper grooming, parasites, and bacterial or fungal infections. All potential causes should be discussed with your veterinarian.
Fragile Skin, Breaking Hair, and Brittle Nails
The final warning sign to watch for is fragility to the skin, hair, and/or nails. Fragility can be a signal that the body is lacking essential nutrients, or that those nutrients are being reallocated elsewhere. Fragile hair will break easily and is prone to matting. Fragile skin tear or abrade easily, even with minimal trauma like normal grooming. Fragile nails may become brittle and fray more easily at the edges.
Genetics can play a large role in the strength of your pet’s skin, hair, and nails. “Normal” can vary from pet to pet. It is important to watch for changes in these tissues as a signal for underlying disease.
Fragility often occurs when critical nutrients – particularly protein and fatty acids – are either 1) poorly absorbed, 2) lost, or 3) reallocated elsewhere in the body. For example, chronic gastrointestinal diseases like inflammatory bowel disease (IBD) can lead to all three mechanisms of fragility. Nutrients are poorly absorbed through an inflamed gastrointestinal tract (GIT). In some cases, proteins are lost from the bloodstream into the GIT (a condition known as protein-losing enteropathy). Lastly, chronic inflammation and damage increase nutrient and energy demand from an already depleted reserve. This chronic inflammation also results in elevated cortisol levels, further contributing to tissue breakdown.
Sources & Further Reading
- Heinrich N. Seborrhea in Animals. Merck Veterinary Manual. (2025). merckvetmanual.com/integumentary-system/seborrhea/seborrhea-in-animals
- Moriello K. Hair Loss (Alopecia) in Dogs. Merck Veterinary Manual. (2025). merckvetmanual.com/.../hair-loss-alopecia-in-dogs
- Vogelnest L. Skin as a Marker of General Feline Health: Cutaneous Manifestations of Systemic Disease. Journal of Feline Medicine and Surgery. 19(9) (2017). doi.org/10.1177/1098612X17723246
