Alopecia in Dogs: Why Dogs Lose Patches of Fur
Noticing a bald patch on your dog where there was once a full, healthy coat can be unsettling. Whether it appears gradually over weeks or seems to develop suddenly, patchy fur loss in dogs — a condition known broadly as alopecia — is a clinical sign rather than a diagnosis in its own right. It can be driven by dozens of different underlying causes, ranging from the straightforwardly treatable to the more complex and chronic. Getting to the root of why your dog is losing fur is the essential first step.
Understanding Alopecia
Alopecia simply means hair loss, and it can manifest in a variety of patterns. Some dogs lose fur in well-defined circular patches; others experience a more diffuse thinning across a large area. Hair loss may be symmetrical — appearing in matching patches on both sides of the body — or entirely asymmetrical. The texture and appearance of the skin beneath the lost fur can also be informative: some alopecia presents on otherwise normal-looking skin, while in other cases the skin is red, scaly, darkened, or thickened.
The pattern, location, and skin appearance of hair loss provide important diagnostic clues for your veterinarian, which is why it is useful to document what you observe before your appointment — including when you first noticed the change and whether it appears to be spreading.
Hormonal Causes of Alopecia
Hormonal imbalances are among the most common causes of symmetrical, non-itchy hair loss in dogs. When the coat is falling out in matching patterns on both sides of the body and the dog does not appear to be scratching or chewing at the affected areas, an endocrine cause should be high on the list of suspects.
- Hypothyroidism — underactivity of the thyroid gland — is one of the most frequently diagnosed hormonal causes of canine alopecia; affected dogs typically also show weight gain, lethargy, and intolerance to cold
- Hyperadrenocorticism, known as Cushing's disease, results from excess cortisol production and commonly causes hair thinning along the flanks, abdomen, and tail, often accompanied by a pot-bellied appearance, increased thirst and urination, and muscle weakness
- Sex hormone imbalances in intact dogs can cause coat changes; in females, oestrogen excess associated with ovarian cysts can produce hair loss along the flanks and perineum
- Alopecia X, sometimes called black skin disease, is a poorly understood condition associated with growth hormone or sex hormone dysfunction, seen most commonly in Nordic breeds such as Pomeranians, Samoyeds, and Keeshonds
Parasitic Causes
Parasites are a significant and often overlooked cause of hair loss in dogs. Mange is caused by microscopic mites that infest the skin and coat, and there are two main types with quite different presentations.
Sarcoptic mange, caused by Sarcoptes scabiei, is intensely itchy and highly contagious — to other dogs and, to a lesser extent, to humans. Hair loss typically begins at the edges of the ears, the elbows, and the face, and the dog will scratch and bite relentlessly at the affected areas. Demodectic mange, caused by Demodex canis, is not contagious and is associated with an immune system that is unable to keep the normal population of Demodex mites in check. It tends to produce localised bald patches, particularly around the face and forepaws, often without significant itching. In some dogs — particularly those that are immunocompromised — it can become generalised and more severe.
Ringworm, despite its name, is not caused by a worm at all but by a fungal infection. It typically produces circular patches of hair loss with a slightly scaly or crusty edge, and it is another condition that can be transmitted to humans.
Allergic and Inflammatory Causes
Dogs with allergies often damage their own coats through repeated scratching, licking, and chewing. Over time, this self-trauma can produce significant hair loss in the areas the dog can reach most easily — the paws, abdomen, groin, axillae, and face. The pattern of hair loss reflects the dog's preferred licking and scratching sites rather than any underlying skin abnormality at those locations.
Folliculitis — bacterial infection of the hair follicles — can cause patchy hair loss with surrounding skin that may appear red, pustular, or crusted. It is often secondary to another skin condition such as allergies or demodicosis, which is why treating the folliculitis alone without addressing the underlying driver frequently leads to recurrence.
Breed-Specific and Genetic Alopecia
Some forms of hair loss are breed-specific and inherited rather than caused by disease. Pattern baldness, or canine pattern alopecia, affects certain breeds including Dachshunds, Chihuahuas, and Miniature Pinschers, typically producing predictable thinning on the ears, temples, chest, and abdomen. Colour dilution alopecia is seen in dogs with dilute coat colours — blue or fawn — and occurs because abnormal melanin pigment deposits damage the hair follicles.
These genetic conditions are not painful and do not affect the dog's quality of life, but distinguishing them from other causes of hair loss is important so that unnecessary diagnostic workups can be avoided.
Investigating Alopecia in Practice
Diagnosing the cause of hair loss in dogs typically involves a combination of the following approaches:
- Skin scrapes examined under a microscope to identify mites
- Fungal culture to detect ringworm
- Blood tests to evaluate thyroid function and screen for Cushing's disease
- Hormonal panels if sex hormone imbalance is suspected
- Skin biopsy for histopathological examination when other tests are inconclusive
- Dietary elimination trials if allergies are a possible contributing factor
Hair loss is one of the most diagnostically complex presentations in veterinary dermatology precisely because so many different conditions can produce similar outward signs. A thorough clinical examination and history are always the starting point, and a systematic approach to ruling out the most common causes will eventually lead to an answer.
Alopecia in dogs is rarely an emergency in itself, but it is almost always a signal that something needs attention. Whether the cause turns out to be a treatable hormonal condition, a parasitic infection, or an inherited trait, understanding why your dog is losing fur puts you in a far better position to support their long-term health and comfort.