Alopecia in Dogs and Cats: Beyond Normal Shedding
Alopecia in dogs and cats signals an underlying disease in most cases. Learn the most common causes of hair loss in pets and how vets diagnose and treat them.
Normal shedding distributes diffusely, follows seasonal patterns, and does not produce visible skin changes. Alopecia is something different: hair loss that produces visible thinning, bare patches, or complete focal baldness, with or without pruritus, scale, or skin color change. The distinction matters because alopecia in dogs and cats is almost never a primary condition. It is a sign, and the underlying cause it points to determines whether treatment requires antifungal medication, hormone supplementation, dietary change, antiparasitic therapy, or immunological management. Treating the hair loss without identifying the cause produces temporary regrowth in some cases and none at all in others. dog skin infections pyoderma guide
What to Know Alopecia in dogs and cats is almost always a sign of an underlying disease rather than a primary condition. The distribution and character of the hair loss, whether it is pruritic or non-pruritic, and whether it involves the follicle or only the hair shaft, point toward the diagnostic category before any testing is performed. Pattern recognition at the first examination narrows a broad differential to two or three leading possibilities.
When Hair Loss Becomes a Diagnostic Signal
The veterinary classification of alopecia begins with two questions that the physical examination answers before any diagnostic test is ordered. First: is the alopecia pruritic or non-pruritic? Second: is the hair being lost or being removed?
Pruritic alopecia indicates inflammation at or around the hair follicle, or a parasitic stimulus that is generating scratching, licking, or barbering sufficient to break or extract hairs. The hair loss is secondary to the itch response. Non-pruritic alopecia indicates either a disease process acting directly on the hair follicle or bulb (endocrine disease, follicular dysplasia, metabolic disturbance) or the complete absence of the hair shaft without the animal needing to interact with the site. These two categories drive different diagnostic approaches.
The second question, whether the animal is losing hair or removing it, is answered by examining the hair shafts at the alopecia margin. Broken hair shafts at uniform length above the follicle opening indicate the animal is contacting the area (scratching, licking, or chewing); the hairs are fractured, not lost from the follicle. Absent hair shafts with a visible empty follicle opening indicate the hair has been shed or failed to grow, not mechanically removed. This distinction is visible on physical examination and is one of the reasons a thorough examination with a magnifying loupe at the alopecia margin changes the differential hierarchy before the first diagnostic test is run.
The distribution pattern adds a third dimension. Symmetrical truncal alopecia without pruritus in a middle-aged dog is an endocrine pattern until proven otherwise. Focal to multifocal alopecia with erythema, scale, and pruritus in a young dog is a parasitic or infectious pattern. Alopecia following the lumbosacral and tail base distribution in a dog with no fleas on its body is a flea allergy dermatitis pattern. Alopecia of the ventral trunk and inner thighs with short, intact-rooted barbered hairs in a cat is an over-grooming pattern. The distribution narrows the differential from the full range of alopecia causes to two or three leading possibilities before a single test is ordered.
What Causes Hair Loss in Dogs?
The most common causes of alopecia in dogs reflect the interaction of immune competence, hormonal status, parasitic exposure, and genetic follicular structure.
Endocrine alopecia is the category most frequently encountered in middle-aged and older dogs. Hypothyroidism, hyperadrenocorticism (Cushing's disease), and sex hormone imbalances (including adrenal sex hormone excess, also called alopecia X) all produce non-pruritic symmetrical truncal alopecia because they alter the hair follicle cycle at the telogen phase: follicles enter telogen and fail to re-enter anagen, producing progressive thinning without replacement growth. hypothyroidism in dogs signs and diagnosis guide
Hypothyroidism produces bilaterally symmetrical truncal hair loss with a classic coat quality change (dry, brittle, hyperpigmented skin at hair loss margins) in a middle-aged dog of a predisposed breed (Golden Retriever, Doberman Pinscher, Irish Setter, Cocker Spaniel). The face and extremities are typically spared. Weight gain without appetite increase and cold intolerance accompany the dermatological signs in most cases.
Hyperadrenocorticism produces truncal alopecia alongside the more systemic signs of glucocorticoid excess: polydipsia, polyuria, pot-bellied abdomen from muscle wasting and hepatomegaly, calcinosis cutis, and hyperpigmentation. The alopecia may be the presenting complaint in mild or early cases before systemic signs are prominent. Cushing's disease in dogs hyperadrenocorticism guide
Parasitic alopecia in dogs is dominated by two mite species: Demodex canis and Sarcoptes scabiei var. canis. Demodex canis inhabits hair follicles in all dogs as a normal commensal; clinical demodicosis develops only when the immune system fails to regulate mite populations, which occurs in young puppies with immature T-cell function, dogs on immunosuppressive medications, or dogs with systemic illness causing secondary immunosuppression. The resulting alopecia is follicular: hairs are lost from inflamed follicles, producing a focal to diffuse pattern with variable pruritus and secondary bacterial folliculitis. Demodex mange in dogs diagnosis and treatment guide
Dermatophytosis (ringworm, caused primarily by Microsporum canis) produces focal circular areas of alopecia with broken hair shafts, scale, and mild erythema. The classic "ringworm ring" is not always present; many cases present as poorly demarcated areas of stubble with scale. Microsporum canis fluoresces apple-green under a Wood's lamp in approximately 50% of cases; a negative Wood's lamp finding does not exclude dermatophytosis.
Citation Capsule: Hypothyroidism is estimated to affect 0.2 to 0.6% of dogs in general veterinary practice and is the most common endocrine cause of symmetrical non-pruritic truncal alopecia in middle-aged and older dogs; accurate diagnosis requires measurement of free T4 by equilibrium dialysis (fT4 ED) alongside TSH, because total T4 is suppressed by concurrent illness, drugs, and breed variation in a significant proportion of euthyroid dogs, leading to false-positive low T4 results; the "euthyroid sick syndrome" causes suppressed total T4 in systemically ill dogs without thyroid disease and is one of the most common causes of misdiagnosed hypothyroidism (Dixon et al., 2002). Free T4 by equilibrium dialysis is more resistant to this effect and should be the primary diagnostic test when hypothyroidism is suspected.
What Are the Most Common Causes of Alopecia in Cats?
Feline alopecia has a different cause distribution from canine alopecia, and the diagnostic approach differs in several important respects. The most important distinction is that cats are obligate over-groomers when pruritic: even mild itching that would produce only mild scratching in a dog may produce intense barbering in a cat, resulting in a near-symmetrical pattern of truncal hair loss that initially suggests a non-inflammatory cause.
Feline symmetrical alopecia is almost always over-grooming related, not follicle-related. Examining the hair shafts at the alopecia margin reveals fractured hairs at a uniform length above the follicle, consistent with barbering. The underlying cause of the barbering is almost always allergic or parasitic: flea allergy dermatitis, food allergy, atopic dermatitis, or Notoedres cati mite infestation.
Flea allergy dermatitis is the most common cause of truncal alopecia, miliary dermatitis, and pruritus in cats. Because cats are fastidious groomers, live fleas are often absent on examination even when flea allergy is the primary driver; flea dirt (flea feces), combed out with a fine-tooth comb and dissolved on a damp white tissue to show a red-brown blood stain, is the most reliable indicator of flea exposure. Strict environmental and on-pet flea control is both diagnostic and therapeutic: improvement during flea control treatment confirms flea allergy.
Food allergy in cats produces pruritus centered on the head, neck, and pinnae (rather than the truncal distribution more typical of dogs), with secondary self-trauma producing alopecia of the face, periocular region, and neck. An 8 to 12-week elimination diet trial is required for diagnosis, identical in protocol to the canine approach.
Hyperthyroidism in older cats (the most common feline endocrine disease) can produce a poor, unkempt coat but rarely produces focal alopecia as a presenting sign. The dermatological changes associated with feline hyperthyroidism are secondary to the systemic metabolic disturbance and include excessive grooming with coat thinning, not classical alopecia.
Citation Capsule: Feline alopecia presenting as symmetrical truncal hair loss is caused by over-grooming secondary to pruritic skin disease in the vast majority of cases rather than by primary follicular disease; examination of hair shafts at the alopecia margin demonstrating fractured hairs (barbering) versus absent hairs from empty follicles (follicular disease) is the critical physical examination finding that distinguishes these two mechanisms and directs the diagnostic approach; flea allergy dermatitis, food allergy, and atopic dermatitis collectively account for the majority of feline pruritic alopecia cases, with true psychogenic alopecia (grooming disorder without underlying dermatological cause) representing a small fraction (Waisglass et al., 2006). Confirming or excluding allergic and parasitic causes before assigning a psychogenic diagnosis prevents long-term diagnostic error.
How Do Vets Diagnose the Cause of Hair Loss in Pets?
The diagnostic workup for alopecia follows the clinical pattern: the distribution and character of the hair loss determine which tests are ordered first.
Alopecia Diagnostic Workup by Presentation Type
| Presentation | First-Line Tests | Key Finding | Next Step |
|---|---|---|---|
| Symmetrical truncal, non-pruritic, middle-aged dog | Total T4, fT4 ED, TSH; LDDS; ACTH stim | Low fT4 + high TSH = hypothyroid; high cortisol = Cushing's | Hormone treatment or further imaging (adrenal) |
| Focal circular, scale, broken hairs | Wood's lamp, fungal culture (DTM), trichogram | Fluorescence or positive culture = dermatophytosis | Systemic antifungal (itraconazole) |
| Multifocal, non-pruritic or mild, young dog | Deep skin scrapings (multiple sites) | Demodex mites in follicular material | Anti-Demodex therapy (fluralaner, moxidectin) |
| Generalized severe pruritus + alopecia | Skin scrapings, flea combing, trial parasiticide | Sarcoptes mites or flea dirt | Trial selamectin or sarolaner; flea control |
| Ventral truncal barbering, cat | Flea combing, DTM culture, elimination diet | Flea dirt / diet response / skin biopsy | Flea control, diet trial, or allergy workup |
| Bilateral flank, seasonal, returning annually | History (season, year), trichogram, biopsy | Telogen follicles in season-appropriate timing | Reassurance; melatonin if desired |
| Post-clipper, dorsal midline only | History of clip + anagen-to-telogen arrest | Telogen hair bulbs at clip site | Spontaneous resolution in 3-6 months |
The trichogram is the single most underused first-line test in the alopecia workup. A trichogram is performed by grasping hairs at the alopecia margin with hemostats, plucking them cleanly, and mounting them in mineral oil for microscopic examination. The findings determine the entire subsequent diagnostic direction: broken or chewed hair shafts with pigment granule leakage confirm mechanical trauma (scratching or barbering); hairs with dystrophic or clubbed telogen bulbs confirm arrest in the telogen phase (endocrine disease or stress telogen effluvium); hairs with swollen anagen bulbs and absent inner root sheaths suggest follicular dysplasia; and hyphae within the hair shaft cortex with arthrospore formation confirm dermatophyte infection. The trichogram takes 5 minutes, costs nothing additional, and changes the diagnostic hierarchy before any expensive test is ordered. It should be the first test in every alopecia workup.
Citation Capsule: Skin biopsy with histopathology is the definitive diagnostic tool for non-inflammatory alopecia when pattern, trichogram, and endocrine testing fail to establish the cause; characteristic histopathological findings distinguish follicular dysplasia (dyskeratotic follicles with distorted inner root sheaths), endocrine alopecia (telogen arrest with flame follicle formation), and alopecia areata (peribulbar lymphocytic infiltrate, "swarm of bees" pattern); biopsy site selection at the active alopecia margin rather than the center of the affected area maximizes diagnostic yield because follicular histopathological changes are most pronounced at the zone of active hair loss progression (Gross et al., 2005). Biopsy at the center of established alopecia frequently shows only secondary atrophic changes that do not reveal the primary cause.
When Is Alopecia a Hormonal Problem?
The category of endocrine alopecia in dogs encompasses several conditions that share a common pathophysiological pathway: disruption of the normal anagen-catagen-telogen hair cycle by hormonal excess or deficiency, causing follicular arrest without the pruritic component that accompanies parasitic and allergic alopecia.
Hypothyroidism is the most frequently diagnosed hormonal cause. Thyroid hormone promotes anagen (growth phase) and is required for normal follicular cycling. Its deficiency causes progressive telogen arrest: follicles enter resting phase and do not re-enter growth phase, producing gradual hair thinning and loss without the normal seasonal cycling that characterizes healthy coat renewal. The absence of pruritus in a dog with bilaterally symmetrical alopecia and a dull, hyperpigmented coat is the clinical combination that mandates thyroid function testing.
Hyperadrenocorticism produces alopecia through excess cortisol, which suppresses normal follicular cycling and simultaneously thins the skin, reduces collagen synthesis, and promotes calcinosis cutis. The pot-bellied appearance from abdominal muscle weakness and hepatomegaly, alongside polyuria and polydipsia, distinguishes Cushing's disease from hypothyroidism at the clinical level in most cases.
Alopecia X (also called adrenal sex hormone alopecia or growth hormone-responsive alopecia) is a condition of unclear pathogenesis affecting primarily Nordic breeds: Pomeranians, Samoyeds, Chow Chows, Alaskan Malamutes, and Keeshonden. It produces progressive truncal alopecia beginning at the base of the tail and advancing cranially, with normal endocrine test results on standard panels. Skin biopsy characteristically shows hair follicle dysplasia and epidermal hyperpigmentation without inflammatory infiltrate. The condition is cosmetically significant but not associated with internal organ disease.
Color dilution alopecia (CDA) is a breed-specific follicular dysplasia occurring in dogs with the dilute color phenotype (blue, fawn, lilac) resulting from homozygous mutation in the MLPH (melanophilin) gene. Abnormal melanin granule distribution within the hair shaft weakens its structure, leading to progressive breakage and thinning beginning in the first two years of life. CDA is not treatable but is not painful; management is supportive with sun protection of the affected skin and monitoring for secondary infections.
The Difference Between Inflammatory and Non-Inflammatory Alopecia
The histopathological classification of alopecia divides causes into two mechanistically distinct groups: inflammatory alopecia, in which the hair follicle is directly attacked by an immune or infectious process, and non-inflammatory alopecia, in which the follicle itself is structurally normal or hormonally arrested.
Inflammatory alopecia encompasses demodicosis (follicular mite burden triggers T-cell infiltration, folliculitis, and furunculosis), bacterial folliculitis (post-inflammatory alopecia with "bull's-eye" lesions and perilesional hyperpigmentation), dermatophytosis (fungal invasion of the hair shaft cortex weakens it mechanically), and alopecia areata (autoimmune attack on anagen bulbs by CD8+ T lymphocytes in a characteristic "swarm of bees" peribulbar infiltrate). In each case, histopathology shows cellular infiltrate at or within the follicle.
Non-inflammatory alopecia encompasses endocrine causes (follicles in telogen arrest without inflammation), pattern alopecia (follicular miniaturization as in human androgenetic alopecia, affecting specific anatomical zones in predisposed breeds), follicular dysplasia (structural abnormality of the inner root sheath or hair shaft), and post-clipping alopecia (telogen arrest induced by clipper blade pressure without follicular damage).
The clinical relevance of this classification is prognostic: inflammatory alopecia resolves with treatment of the cause (antimicrobials for folliculitis, antifungals for dermatophytosis, anti-Demodex therapy for demodicosis), while non-inflammatory alopecia resolves only when the hormonal abnormality is corrected (hypothyroidism responds to levothyroxine with coat regrowth in 4 to 6 months) or not at all (pattern alopecia, color dilution alopecia, alopecia X).
Can Stress and Psychogenic Factors Cause Hair Loss in Cats?
Psychogenic alopecia in cats refers to compulsive over-grooming driven by anxiety, stress, or behavioral disturbance rather than pruritus. It exists as a diagnosis. What is clinically problematic is how frequently it is assigned as the first-line explanation for feline truncal alopecia without systematic exclusion of dermatological causes.
What veterinary dermatologists observe consistently: Feline psychogenic alopecia is significantly overdiagnosed in general practice. When a cat presents with ventral truncal barbering and the owner reports a household stressor (new baby, new pet, recent move, construction noise), psychogenic alopecia is often offered as the diagnosis. In a published series, approximately 75% of cats initially diagnosed with psychogenic alopecia were found on systematic dermatological investigation to have an underlying allergic or parasitic cause, most commonly flea allergy dermatitis or food allergy. These cats were pruritic; the pruritus was subtle enough that owners interpreted the grooming as behavioral rather than symptom-driven. Psychogenic alopecia exists, but it is a diagnosis of exclusion that requires: a negative flea combing and response trial, a negative elimination diet response, a negative response to anti-pruritic medication trial, and ideally a skin biopsy showing no inflammatory infiltrate. Assigning it without completing these steps leaves the underlying dermatological cause untreated.
The pruritic threshold in cats is relevant here: cats may groom excessively in response to pruritus intensities that would produce only mild scratching in a dog. A cat with subclinical flea allergy whose pruritus score would be rated as 2 out of 10 in a dog may groom itself ventral alopecia in 6 weeks. The owner sees no visible scratching, only a cat that sits quietly and licks. The behavior appears calm and voluntary. The diagnosis of psychogenic alopecia is assigned. The flea treatment is not initiated. The pattern continues indefinitely. food allergies in pets elimination trial guide
Genuine psychogenic alopecia does respond to behavioral enrichment and in refractory cases to pharmaceutical anxiolysis (fluoxetine, clomipramine), but initiating treatment without dermatological exclusion wastes months while the animal's pruritic underlying condition continues untreated.
Frequently Asked Questions
Why is my dog losing hair in patches?
Patchy hair loss in dogs is most commonly caused by one of three mechanisms: folliculitis and furunculosis from Staphylococcus bacterial infection (producing circular "target lesion" alopecia with perilesional hyperpigmentation), Demodex mite infestation of hair follicles (producing irregular patchy alopecia beginning on the face and paws in young dogs), or dermatophytosis (producing circular areas with broken hairs, scale, and mild erythema). A skin scraping, trichogram, and fungal culture ordered together during a single appointment address all three possibilities simultaneously.
What causes cats to lose hair on their belly?
Ventral truncal alopecia in cats is almost always caused by over-grooming secondary to pruritus. The most common underlying causes are flea allergy dermatitis (even with no visible fleas on the cat, flea dirt on a white tissue confirms exposure), food allergy, and atopic dermatitis. Psychogenic alopecia is a less common cause that should be diagnosed only after allergic and parasitic causes are excluded. Examining the hair shafts at the alopecia margin reveals fractured (barbered) hairs in over-grooming cases rather than the empty follicle openings seen in true follicular hair loss.
Is alopecia in dogs permanent?
Permanence depends entirely on the cause. Endocrine alopecia (hypothyroidism, Cushing's disease) reverses with appropriate hormone treatment, with coat regrowth typically visible within 3 to 6 months. Folliculitis-related alopecia resolves with antimicrobial treatment. Color dilution alopecia, pattern alopecia, and some cases of alopecia X do not reverse and represent permanent or progressive conditions. Follicular dysplasia and alopecia areata may be permanent depending on the extent of follicular damage.
Can seasonal shedding cause bald patches in dogs?
Normal seasonal shedding produces diffuse coat thinning without focal bald patches and does not produce skin color changes or scale. Focal bald patches, regardless of season, represent a condition requiring investigation. Seasonal flank alopecia (which produces bilateral flank patches on a predictable annual schedule) is a recognized condition distinct from normal shedding; it regrows spontaneously each year and follows the photoperiod cycle, recurring in the same season annually. No other pattern of focal alopecia should be attributed to normal shedding without investigation.
When should I take my cat to the vet for hair loss?
Any cat with hair loss visible to the naked eye on a single examination should be evaluated. Cats hide pruritus; the absence of visible scratching does not mean the animal is not pruritic. Hair loss in cats is almost always caused by over-grooming secondary to pruritic skin disease, and untreated pruritic skin conditions progress with ongoing secondary self-trauma. Early evaluation prevents escalation of secondary skin trauma and allows the underlying cause to be identified before the pattern becomes chronic.
The Pattern That Points to the Cause
The alopecia workup in dogs and cats begins not with a test but with an observation: where is the hair lost, what do the remaining hair shafts look like at the margin, and is the animal interacting with the site? These three observations, available in the first two minutes of the physical examination, reduce a differential list of thirty possible causes to two or three. The diagnostic tests that follow are confirmation, not exploration. The pattern was always there at the first examination, pointing at the answer for anyone looking at it with the right framework.
