SYMPTOM
Hair loss or coat changes
Thinning fur, bald patches, or changes in coat texture and quality that develop over weeks to months.
Endocrine
Hormonal imbalances are among the most common systemic causes of coat changes and hair loss in companion animals. Hypothyroidism, hyperadrenocorticism, and sex hormone imbalances can all alter the hair growth cycle, sebum production, and skin quality. Endocrine-related alopecia tends to be bilaterally symmetrical, non-pruritic, and may affect the trunk while sparing the head and extremities. The coat may become dry, dull, or brittle, and hair that is lost may be slow to regrow or may not regrow at all until the hormonal imbalance is addressed.
Allergic or Immune-Mediated
Allergic skin disease can produce hair loss through a combination of inflammation, self-trauma from scratching and licking, and disruption of normal follicular function. Environmental allergens, food sensitivities, and flea allergy dermatitis may all contribute to coat changes, with the distribution of hair loss often reflecting the pattern of allergic inflammation and self-trauma. Unlike endocrine causes, allergic hair loss is typically accompanied by pruritus and may show seasonal variation if environmental allergens are involved.
Infectious or Parasitic
Bacterial skin infections, fungal infections such as dermatophytosis, and parasitic infestations including demodecosis and sarcoptic mange can all produce hair loss with varying patterns and characteristics. Dermatophytosis often produces circular areas of alopecia with scaling at the margins, while demodicosis may cause patchy or generalised hair loss with secondary bacterial infection. Parasitic causes may produce intense pruritus leading to self-inflicted hair loss, or may directly damage hair follicles.
Nutritional
Deficiencies in essential fatty acids, zinc, biotin, or protein can manifest through changes in coat quality before other signs of nutritional inadequacy become apparent. The coat may become dry, brittle, or lose its normal lustre, and hair growth may slow or thin. These nutritional influences can also reduce the skin's natural barrier function, potentially predisposing to secondary infections. In most cases involving commercial pet foods, nutritional causes are less common than other factors, but may be relevant with home-prepared diets or conditions affecting nutrient absorption.
Stress or Psychogenic
Chronic stress, anxiety, or compulsive behaviour can lead to excessive grooming that produces hair loss, particularly in cats. Psychogenic alopecia typically affects areas the animal can easily reach — the abdomen, inner thighs, and forelimbs — and the hair may be broken rather than absent at the follicular level. Distinguishing between psychogenic over-grooming and grooming driven by an underlying dermatological or systemic condition can be challenging, as both produce similar patterns of hair loss.
Neoplastic
Certain tumours, both cutaneous and systemic, can produce paraneoplastic skin changes including alopecia and coat quality deterioration. Sertoli cell tumours may cause bilaterally symmetrical alopecia through hormonal effects, while paraneoplastic alopecia in cats has been associated with internal neoplasia. These causes are relatively uncommon but may be considered when coat changes are accompanied by other systemic signs or when the pattern does not fit more typical categories.
Why timing matters
Early observation
Early coat changes may manifest as subtle shifts in texture, lustre, or density rather than obvious hair loss. The coat may feel drier, appear duller, or seem to lack its usual volume. Increased shedding beyond seasonal norms, a coat that does not regrow as quickly after clipping, or patches where the undercoat seems thinner may be among the first changes noticed. At this stage, the changes may be dismissed as seasonal variation, the effects of bathing products, or simply ageing, but they can represent the earliest visible manifestation of an underlying process that has been developing for some time.
Later presentation
As coat changes progress, they typically become more obvious and may include frank areas of alopecia, widespread thinning, changes in hair colour or texture, or the development of secondary skin changes such as hyperpigmentation, scaling, or comedone formation. The skin underlying areas of hair loss may appear thickened, darkened, or abnormally thin depending on the cause. At this stage, secondary infections may develop in compromised skin areas, adding another layer of change. The animal's overall appearance may be notably altered from its previous normal, and the changes are unlikely to resolve without addressing the underlying cause.
The progression of coat changes often mirrors the trajectory of the underlying condition. Endocrine causes tend to produce slowly progressive, symmetrical changes over months. Allergic causes may fluctuate with seasons or exposures but tend to worsen over time without management. Infectious causes may spread from an initial focal area if untreated. Nutritional causes may stabilise or improve with dietary modification but can also worsen if the underlying deficiency deepens. The rate and pattern of progression, combined with the distribution of changes and the presence or absence of pruritus, can help characterise the type of process at work.
Conditions commonly associated
Hypothyroidism in Dogs
Cushings Disease in Dogs
Pyoderma in Dogs
Hair loss or coat changes may accompany pyoderma as bacterial infection damages hair follicles, leading to patchy alopecia in affected areas.
Exocrine Pancreatic Insufficiency (EPI)
Poor coat quality, dullness, and hair changes can develop in EPI due to chronic malabsorption of essential fatty acids and fat-soluble vitamins necessary for healthy skin and coat maintenance.
Feline Immunodeficiency Virus (FIV)
Coat changes and hair loss in FIV-positive cats may result from dermatophyte infections, demodicosis, or other skin conditions that gain a foothold as cell-mediated immune responses diminish.
Flea Allergy Dermatitis
Self-trauma from scratching and overgrooming frequently causes patchy hair loss, particularly along the back and hindquarters.
Acute Moist Dermatitis (Hot Spots)
Hair loss over the lesion occurs from self-trauma and is often accentuated by clipping for treatment.
Dermatophytosis (Ringworm)
Dermatophytosis characteristically causes circular or irregular patches of hair loss as the fungus damages hair shafts and follicles.
Demodectic Mange
Hair loss is a hallmark of demodectic mange, typically appearing as patchy alopecia around the face, feet, or spreading more widely in generalised cases. The hair loss often has a moth-eaten appearance.
Sarcoptic Mange
Progressive alopecia develops as scratching and inflammation damage hair follicles, typically affecting ear margins, elbows, hocks, and ventral areas.
Sebaceous Adenitis
Hair loss and coat changes in sebaceous adenitis result from destruction of sebaceous glands and subsequent follicular dysfunction.
Pyoderma
Bacterial skin infection can damage hair follicles and cause patchy hair loss in affected areas, sometimes with characteristic circular patterns.
Cryptorchidism
Feminisation from oestrogen-producing tumours causes symmetrical hair loss.
Pemphigus
Affected areas lose fur as the skin becomes damaged, crusted, and eroded.
Acral Lick Dermatitis
The licked area loses fur due to constant moisture, friction, and damage to hair follicles.
When to explore further
Bilaterally symmetrical hair loss affecting the flanks, trunk, or tail base, particularly without accompanying itchiness, may suggest a systemic or hormonal influence on the hair growth cycle rather than a localised skin condition, and this distribution pattern carries different implications from patchy or asymmetric loss.
When coat changes are accompanied by other systemic signs such as changes in weight, energy levels, appetite, thirst, or urination patterns, the combination may suggest that the coat changes are one manifestation of a broader metabolic or endocrine process affecting multiple body systems.
Hair loss that is accompanied by visible skin changes — redness, scaling, crusting, pustules, or darkened pigmentation — may suggest an active inflammatory or infectious process in the affected skin rather than a purely hormonal or nutritional cause of follicular dormancy.
Coat changes that begin after a medication change, dietary modification, or environmental exposure may have a temporal relationship with the new factor, and noting the timing of onset relative to these changes can provide useful contextual information.
In cats, hair loss affecting the ventral abdomen and inner thighs that appears to have been caused by grooming — with broken hair shafts rather than completely empty follicles — may reflect either a dermatological condition driving excessive grooming or a behavioural pattern, and the distinction can influence understanding of the underlying cause.
Documenting coat changes over time — through photographs taken in consistent lighting and from consistent angles — can provide a valuable visual record of progression that is difficult to assess through memory alone. Noting the distribution of changes, whether the affected areas are expanding, and whether the animal appears to be actively contributing to the hair loss through scratching or licking can help characterise the process. Observing how the coat responds to seasonal changes, dietary modifications, or changes in the animal's environment may also reveal patterns that contribute to understanding the underlying factors at play.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS