Dog Hair Loss (Alopecia) — Symptom Guide

How to recognize types of hair loss in dogs, likely causes (hypothyroidism, Cushing's, allergies, parasites, ringworm), basic home checks, red flags, and when to see a vet.

Quick Assessment

• Is this an emergency? No — usually not. Yes if any of: high fever (>104°F/40°C), severe lethargy, breathing difficulty, rapid widespread loss over 24–48 hours, or evidence of systemic illness. (See “When It's an Emergency” below.)
• Most common cause: allergy/flea-related skin disease and parasitic infections (fleas, mites), and secondary bacterial skin infections.
• When to see a vet: If hair loss is spreading, you see open sores/pustules, the dog is very itchy, there are systemic signs (appetite change, vomiting, excessive drinking/urination), or the problem lasts >2 weeks despite home measures.

What this symptom looks like

Hair loss (alopecia) in dogs can take many forms:

• Thinning coat over a large area, often symmetrical across the trunk.
• Clearly defined bald patches, which may be round, irregular or spreading.
• Broken hairs, scaling (dandruff), redness, crusts, pustules or a greasy coat.
• Signs the owner may notice first: more shedding than usual, visible skin, scabs, rubbing or chewing at a spot.

Likely causes (ranked common → less common)

(References: Merck Veterinary Manual — Dermatology, Hypothyroidism, Hyperadrenocorticism, Demodicosis, Dermatophytosis)

Pattern recognition — quick rules of thumb

• Symmetrical, non-itchy thinning over flanks/trunk, poor hair regrowth → think hormonal (hypothyroidism, Cushing's).
• Patchy, circular areas with scaling/crusting → think ringworm or focal bacterial infection.
• Intense pruritus with small red bumps, hair loss at base of tail, inner thighs, ears → think fleas or allergic dermatitis.
• Young dog with one or a few small patches, no strong itch → localized demodex or ringworm.
• Widespread pustules, foul odor, and fever → think deep pyoderma or severe infection (urgent).

Decision tree — if [symptom] + [other sign] → likely [cause] → [action]

• If patchy circular alopecia + scaly edges + sometimes broken hairs → likely ringworm → action: isolate pet, avoid handling without gloves, call vet for fungal culture or topical/systemic therapy.
• If severe itching + fleas or flea dirt visible + hair loss at rump/tail base → likely flea allergy dermatitis → action: start veterinary-grade flea control immediately and schedule vet visit if skin looks infected.
• If symmetrical thinning over trunk + weight gain + cold intolerance + slow heart rate → likely hypothyroidism → action: schedule vet visit for thyroid (TT4, free T4, TSH) and baseline bloodwork.
• If symmetrical thinning + increased drinking/urination + pot-bellied appearance + thinning skin → likely Cushing's (hyperadrenocorticism) → action: schedule vet for endocrine testing (urine cortisol:creatinine, ACTH stimulation or low-dose dex suppression per vet guidance).
• If localized patch in puppy + minimal itch + young dog under 1 year → likely localized demodex vs ringworm → action: vet exam with deep skin scrapings and/or fungal culture.
• If pustules, widespread redness, fever >103–104°F, painful areas → likely severe infection → action: urgent vet evaluation (within 24 hours) or emergency clinic.

Home assessment steps (what to check and measure)

Diagnostic approach your vet will likely use

• Physical exam and history.
• Skin cytology (tape prep, impression smears) to look for bacteria or yeast.
• Superficial and deep skin scrapings for mites (e.g., Sarcoptes, Demodex).
• Fungal testing (Wood's lamp as rough screen, but fungal culture on dermatophyte test media is the gold standard).
• Bacterial culture if deep or non-responsive infections.
• Skin biopsy in unclear or chronic cases.
• Bloodwork: CBC, chemistry profile; endocrine tests (TT4/free T4/TSH for hypothyroidism; urine cortisol:creatinine ratio, and ACTH stimulation or low-dose dexamethasone suppression test for Cushing's) when indicated.

When It's an Emergency — red flags

Seek immediate veterinary care or go to an emergency clinic if any of the following are present:

• Difficulty breathing, collapse, severe weakness.
• High fever >104°F (40°C) or persistent fever >103°F (39.5°C) with lethargy.
• Rapidly spreading skin lesions over 24–48 hours.
• Large open sores, bleeding, or signs of severe pain.
• Signs of systemic illness (vomiting, refusing to eat for >24–48 hours, severe panting, sudden increased drinking/urination in combination with other signs).

When to Schedule a Vet Visit (non-urgent but needs attention)

Make an appointment within 48–72 hours if any of these apply:

• Hair loss is spreading or not improving and has lasted >2 weeks.
• Dog is very itchy (frequent scratching, rubbing, chewing) or the skin looks infected (pustules, oozing, bad smell).
• There are signs of endocrine disease (weight gain or loss, changes in drinking/urination, hair thinning in a symmetrical pattern).
• Multiple pets or people have similar skin lesions (possible contagious disease).
• You found fleas, many ticks, or suspect a parasite.

Safe Home Care while monitoring

• Isolate the animal from other pets and people if you suspect ringworm; wear gloves and wash hands.
• Do not apply human topical steroids or antifungals without veterinary advice — these can make some infections worse.
• For mild itching: bath with a veterinary-recommended gentle shampoo (chlorhexidine or oatmeal as advised by your vet) every 7–14 days; thoroughly dry skin.
• Use a properly labeled, vet-approved flea control product (dog-specific) and treat all in-contact pets; do not use cat products on dogs unless labeled.
• Prevent self-trauma: use an Elizabethan collar if your dog is chewing or rubbing a lot.
• Clean bedding and vacuum frequently; for ringworm, launder bedding and disinfect surfaces.
• Avoid topical over-the-counter treatments unless your veterinarian recommends them.

What to tell your vet (helpful, targeted information)

Prepare this information to speed diagnosis:

• Exact onset date and progression (photos are very helpful).
• Itchiness score (0–10) and whether your dog is licking/chewing/rubbing more than usual.
• Any other signs: appetite change, thirst/urination changes, vomiting, diarrhea, coughing, difficulty breathing.
• Medications and supplements taken in the last 3 months (including topical flea/tick products and steroids).
• Recent grooming, shampoos, topical products, new foods, treats, or household cleaners.
• Exposure to other animals (boarded, shelter, new dogs/cats) and whether any other pet or person has similar lesions.
• Flea/tick prevention used (brand, last dose), travel history, and age at first signs.

Prognosis and expectations

Outcome depends on cause: parasite- and fungal-related hair loss often resolves with appropriate treatment and time for hair regrowth (weeks to months). Hormonal causes require long-term medical management. Secondary bacterial infections frequently resolve quickly once the underlying problem is treated.

Remember: this guide helps you assess urgency and gather useful information, but only your veterinarian can perform the diagnostic tests required to determine the underlying cause and safe treatment plan. Avoid guessing or starting prescription medications without professional guidance.

Sources

• Merck Veterinary Manual — Dermatology and specific pages on Hypothyroidism, Hyperadrenocorticism, Demodicosis, Dermatophytosis: https://www.merckvetmanual.com

Frequently Asked Questions