SYMPTOM
Skin thinning or bruising
Skin that appears fragile, tears more easily, or shows bruising without obvious external trauma.
Endocrine (Hyperadrenocorticism)
Excess cortisol production is one of the most well-recognised causes of skin thinning and easy bruising in dogs and cats. Chronically elevated cortisol levels affect the skin's collagen structure, reducing its thickness and elasticity while also weakening blood vessel walls. The skin may become visibly thinner, sometimes to the point where underlying blood vessels are visible through the surface, and bruises may appear after handling that would not normally cause injury. These changes typically develop alongside other signs of cortisol excess such as increased thirst, a pot-bellied appearance, and coat changes.
Coagulation Disorders
Abnormalities in the blood's clotting mechanisms can produce easy bruising even with normal skin thickness. Conditions affecting platelet numbers or function, clotting factor deficiencies, and diseases that consume clotting components can all result in bruising that appears disproportionate to the level of trauma involved. Platelet problems typically show as pinpoint red spots (petechiae), larger bruises (ecchymoses) and bleeding from the gums or nose. Clotting factor deficiencies, such as those caused by anticoagulant rat poison, can also cause swellings of blood under the skin and bleeding into the chest or abdomen, which cannot be seen from outside. In the UK, lungworm (Angiostrongylus vasorum) is a further cause of bleeding in dogs.
Medication Effects
Long-term corticosteroid treatment can produce skin thinning and easier bruising as side effects. Some other drugs can suppress platelet production in the bone marrow, which causes bruising by a different route. Corticosteroids mimic the effects of endogenous cortisol on skin structure, and prolonged use can produce changes similar to those seen in naturally occurring hyperadrenocorticism. These iatrogenic skin changes may develop gradually during treatment and can vary depending on dose and duration.
Hepatic
The liver produces many of the proteins involved in blood clotting, and significant liver dysfunction can impair the production of clotting factors, leading to increased bruising tendency, and bleeding into body cavities from this can cause sudden collapse. Animals with advanced liver disease may show bruising alongside other signs of hepatic compromise such as jaundice, fluid accumulation, and changes in appetite.
Immune-Mediated
Immune-mediated conditions can target platelets or other components of the clotting system, producing bruising through immune-mediated destruction of these blood components. Immune-mediated thrombocytopenia, where the body's immune system attacks its own platelets, is a relatively well-recognised cause of spontaneous bruising in dogs. The bruising may appear suddenly and can affect multiple body areas, sometimes producing dramatic discolouration.
Why timing matters
Early observation
Early skin thinning may be difficult to detect, manifesting as a subtle change in skin texture or an increased tendency for minor scratches or abrasions to occur during normal handling. The skin may feel papery or fragile compared with its previous texture, and small bruises may appear in areas subject to everyday contact. These early changes can develop over weeks to months and may initially be attributed to ageing or normal variation.
Later presentation
As skin thinning progresses, the changes become more visually apparent. The skin may appear translucent, with blood vessels clearly visible beneath the surface, particularly over the abdomen and inner thighs. Bruising may occur with minimal provocation, and the skin may tear easily during handling, grooming, or blood sampling. Hair loss over affected areas may further expose the altered skin, creating a distinctive appearance.
The rate at which skin thinning and bruising develop depends on the underlying cause. Endocrine-driven changes typically progress gradually over months, while medication-related effects may develop more predictably in relation to treatment duration and dose. Coagulation disorders, including immune-mediated destruction of platelets, can produce bruising over hours to days, and these are the causes in which bleeding inside the body can become life-threatening. Tracking the locations, size, and frequency of bruises suits the slower skin changes; bruising that appears over hours to days cannot be characterised at home, because it needs a platelet count and clotting tests.
Conditions commonly associated
Cushings Disease in Dogs
Skin thinning and bruising in Cushing's disease occur because chronically elevated cortisol breaks down collagen and weakens the supportive connective tissue beneath the skin, leaving it fragile and blood vessels more exposed to injury.
Pemphigus
Pemphigus involves autoimmune disruption of the connections between skin cells, which can lead to fragility, blistering, and erosions that may be mistaken for bruising or thinning, particularly when crusts and superficial tissue loss are present.
Cushings Disease in Cats
Excess cortisol thins the skin and blood vessel walls, making the skin fragile, easily bruised and prone to tearing.
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Anticoagulant Rodenticide Toxicity
Without functioning clotting factors, minor knocks can produce bruising that spreads more than would normally be expected under the skin.
Atypical Cushings in Dogs
Hormonal effects on the skin's collagen leave it thinner and more prone to bruising than normal.
Calcinosis Cutis
Calcinosis cutis often develops alongside excess steroid exposure, which itself thins the skin and leaves it prone to bruising or tearing.
Vasculitis
Vasculitis damages the walls of small blood vessels, producing purple, bruise-like patches (purpura) and in some cases sharply edged ulcers, most often on the ear flaps, tail and footpads.
Angiostrongylus (Lungworm)
Lungworm infection can disturb blood clotting in dogs: in a study of 180 infected dogs, about a third (36%) showed bleeding, and survival to discharge was lower in those that bled (77% against 95%).
When to explore further
Bruising that appears without identifiable trauma, or that seems disproportionate to the level of handling or activity involved, may suggest that the skin or clotting system is compromised. Spontaneous bruising, where discolouration appears without any recalled injury, is particularly noteworthy.
Skin thinning that develops alongside other systemic changes such as increased thirst and urination, abdominal distension, muscle wasting, or coat changes may suggest an endocrine process affecting multiple body systems simultaneously. The combination of skin changes with these additional signs can be more informative than the skin changes alone.
Easy bleeding from minor wounds, prolonged bleeding from cuts, or blood in the urine or stool occurring alongside bruising may suggest a more generalised problem with blood clotting rather than skin thinning alone. These additional bleeding signs can provide important context about the nature of the underlying disorder. Pale gums, weakness, laboured breathing or collapse alongside bruising can mean bleeding inside the body.
Bruising that develops acutely, with multiple bruises appearing over hours to days in a previously unaffected animal, may suggest an acute-onset clotting disorder that differs from the gradual progression of endocrine or medication-related skin changes. The speed of onset can help distinguish between different categories of causes. Possible access to anticoagulant rat poison is relevant here, because visible bleeding typically appears 3 to 7 days after a toxic dose is eaten.
Skin thinning that develops over weeks to months of corticosteroid treatment, or bruising that appears after starting any new medication, may represent a medication-related effect. The temporal relationship between medication introduction and the development of skin changes can be informative.
For slowly developing skin changes, photographing areas of bruising or skin thinning with a size reference provides documentation that can be compared over time and shared with veterinary professionals. Noting the location, size, and any recalled precipitating cause for each new bruise helps build a record that may reveal patterns. Observing the gum colour and looking for pinpoint reddish spots on the gums, inner ear flaps, or belly shows whether the bleeding tendency extends beyond the skin, which points towards a clotting problem rather than skin thinning. Handling the animal gently and avoiding rough grooming or play that might cause skin tears helps reduce the risk of injury while the underlying cause is being explored.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS