Why is my dog losing hair?

Split the question first: does the bald area itch? Itchy hair loss usually means parasites, infection or allergy, and the hair is coming out because the dog is removing it. Non-itchy hair loss, especially symmetrical on both sides of the body, points instead at hormonal disease such as hypothyroidism or hyperadrenocorticism, where the hair is falling out on its own.

What does symmetrical hair loss on a dog mean?

Bilaterally symmetric, non-itchy hair loss over the trunk and flanks is the classic presentation of endocrine disease. Canine endocrine dermatopathies account for roughly 8.6% of dermatology appointments, most commonly hypothyroidism, hyperadrenocorticism and hyperoestrogenism. A 'rat tail' appearance, hair loss along the tail, is a recognised hypothyroid sign.

Is my dog's hair loss mange?

It might be, and there are two very different kinds. Sarcoptic mange is intensely itchy, highly contagious and can affect humans in the household. Demodectic mange is usually not very itchy, is not contagious, and in an adult dog appearing for the first time it can signal an underlying immune or hormonal problem. Both are diagnosed by skin scrapings, though sarcoptic mites are frequently missed even when present.

Is heavy shedding the same as hair loss?

No. Shedding thins the coat evenly all over and leaves no bald skin. Hair loss leaves visible patches or areas of exposed skin. If you can see skin, that is alopecia and it needs investigating, however much hair is coming off on the sofa, normal shedding does not produce bare patches.

What tests will the vet run for hair loss?

Typically skin scrapings for mites, tape or swab cytology for yeast and bacteria, and a fungal culture or PCR if ringworm is suspected. If the pattern is non-itchy and symmetrical, expect bloodwork including thyroid testing and screening for hyperadrenocorticism. In stubborn or unusual cases, a skin biopsy.

Dog Losing Hair? The First Question Is Whether It Itches

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There is one question that sorts most hair loss into the right half of the problem, and it takes a day of watching to answer.

Does it itch?

Itchy hair loss means the hair is being removed, scratched, chewed, licked or rubbed away by the dog. Non-itchy hair loss means the hair is falling out on its own, and that usually points inward, at hormones, rather than at the skin.

Those are two different work-ups. Starting with the wrong one costs months.

Itchy hair loss

The hair is a casualty of the scratching. Find what is causing the itch and the coat follows.

Parasites first, always.

Fleas, hair loss over the tail base, rump and back of the thighs is close to a signature
Sarcoptic mange, intensely itchy, ear margins, elbows, hocks and belly, contagious to other dogs and to people in the house. Frequently missed on skin scrapings even when the dog is full of mites, so many vets reasonably treat on suspicion
Demodectic mange, usually not very itchy unless secondarily infected. Common in puppies as localised patches, often around the face and forelegs, and frequently self-resolving. In an adult dog appearing for the first time, generalised demodicosis is a red flag for an underlying immune or hormonal problem, and it deserves a broader investigation than a treatment for mites

Infection. Bacterial folliculitis produces expanding circular patches of hair loss with crusted edges, often confused with ringworm. Yeast produces greasy, darkened, musty-smelling skin.

Ringworm. A fungal infection, not a worm. Circular patches, and it is transmissible to humans, worth confirming rather than assuming, with a culture or PCR.

Allergy. Rarely causes hair loss directly; causes the itching that does. Work it in the proper order.

Non-itchy hair loss

This is the half most often mishandled, because it looks like a skin problem and is not one.

The pattern that matters: bilaterally symmetric. Both flanks, both sides of the trunk, evenly, without the dog paying it any attention. Non-inflammatory alopecia tends to be generalised or bilaterally symmetric, and is often non-pruritic.

Endocrine dermatopathies make up around 8.6% of dermatology appointments, most commonly:

Hypothyroidism. Non-itchy, bilaterally symmetric hair loss affecting mainly the lateral trunk and flanks, and a "rat tail" appearance as hair is lost from the tail. Often with thinning coat, darkened skin, thickened skin, lethargy, weight gain and recurrent ear infections.

Hyperadrenocorticism (Cushing's). Generalised symmetric non-inflammatory hair loss over the trunk, often alongside increased drinking and urination, a pot-bellied appearance, thin skin, panting, and comedones, blackheads, over the trunk.

Hyperoestrogenism. Usually associated with a hormone-producing tumour; in males, an enlarged testicle alongside symmetrical hair loss is the classic combination.

An important overlap: both Cushing's and hypothyroidism can trigger demodicosis and secondary bacterial or yeast infection, because both suppress immune function. So a dog can present with the itchy picture and have an endocrine disease underneath it. If a dog keeps getting skin infections, or develops adult-onset demodicosis, the question is what is allowing them, not just how to clear this episode.

Other non-itchy causes: post-clipping alopecia, seasonal flank alopecia, pressure-point calluses, and injection-site reactions.

Shedding is not hair loss

Worth separating, because a lot of worried owners have a perfectly normal dog.

	Shedding	Hair loss
Coat	Thins evenly overall	Patches, or defined areas
Skin	Not visible	Visible
Pattern	All over	Localised or symmetric
Dog's interest	None	Scratching, if itchy type

If you can see skin, it is alopecia. However much hair is coming off on the furniture, normal shedding does not produce bare patches.

Indian shedding runs heavier and longer than the seasonal pattern most Western sources describe, a warm climate and artificial indoor lighting keep double-coated dogs shedding much of the year. That is normal, and it is not what this article is about.

What to bring to the vet

Photographs, weekly, from the same angles. Hair loss progresses slowly enough that you cannot judge it from memory, and a series of photographs is genuinely diagnostic.

Answer these before you go:

Does it itch? (The whole first fork depends on this)
Symmetrical, or patchy and random?
Where did it start?
Any other dogs or people in the house itching?
Drinking more? Urinating more? Panting more?
Weight change? Energy change?
Any recent clipping, injection, or new medication?

Tests to expect: skin scrapings for mites, cytology for yeast and bacteria, fungal culture or PCR if ringworm is a possibility. If the picture is non-itchy and symmetrical, bloodwork including thyroid testing and screening for hyperadrenocorticism. Skin biopsy for stubborn or unusual cases.

The bottom line

Ask whether it itches. That question splits hair loss into two different diseases with two different investigations, and it costs you nothing but a day's observation.

Itchy and patchy points at the skin. Non-itchy and symmetrical points at hormones, and a dog with a bald flank on each side and a rat-like tail should be having a blood test, not another medicated bath.

Adult-onset demodicosis, or a dog that keeps developing skin infections, is a question about what is allowing them, often endocrine, rather than about how to clear this episode.