CONDITION
Nephrotic Syndrome
Nephrotic syndrome describes a pattern in which the kidneys lose large amounts of protein into the urine, enough to lower protein levels in the bloodstream and trigger a cascade of changes throughout the body. This protein loss is typically caused by damage to the filtering units within the kidney, though the underlying reason for that damage can vary. Owners often first notice swelling—around the abdomen, under the skin, or sometimes around the legs—alongside changes in appetite, energy, or the appearance of the urine itself. In some cases, the condition is identified through routine blood or urine tests before visible signs appear. The swelling and other features arise not from kidney failure in the traditional sense, but from the body's response to losing protein it normally retains. This page explores the signs that can suggest nephrotic syndrome, the mechanisms that drive protein loss and fluid shifts, the investigations used to characterise the pattern and search for underlying causes, and the range of approaches available depending on what is found.
Why this matters now
Nephrotic syndrome can appear at any age, though certain inherited forms tend to emerge in young adults, whilst other patterns may develop in middle-aged or older animals. Some breeds carry a higher risk of specific glomerular diseases that lead to this pattern, particularly soft-coated wheaten terriers, English cocker spaniels, and certain bull terrier lines. In other cases, the protein loss follows an immune-mediated process or exposure to infectious agents, with no clear breed or age boundary.
The onset of protein loss may be gradual, with blood tests showing changes before the owner notices anything at home, or it may declare itself more abruptly with visible swelling over the course of days to weeks. Some animals stabilise with modest protein loss that persists for months or years, whilst others experience a more aggressive decline in kidney function or develop complications such as blood clots. The trajectory often depends on the underlying cause and how the filtering units respond over time.
Signals & patterns
Early signals
Abdominal distension or fullness
Fluid may begin to accumulate in the abdomen, creating a rounded or pendulous appearance that differs from weight gain. The belly may feel softer or more fluid-filled when gently pressed, and the change can develop over days to weeks.
Puffiness under the skin
Swelling may appear in areas where skin is loose—under the jaw, along the chest, or around the lower legs. The tissue often holds a gentle indent when pressed, a sign that fluid has shifted into the spaces between cells.
Foamy or frothy urine
Urine that contains large amounts of protein may form bubbles or foam when it hits the ground or the litter tray. This sign can be subtle and is often noticed only in hindsight, once other changes prompt closer attention.
Reduced appetite or energy
Some animals become quieter or show less interest in food as the body adjusts to low blood protein levels and fluid shifts. The change may be gradual and easy to attribute to other causes at first.
Later signals
Laboured breathing or rapid breaths
Fluid can collect in the chest cavity or within the lungs themselves, making each breath feel heavier. This often develops when protein levels drop further or when complications such as blood clots affect circulation.
Sudden limb swelling or pain
Blood clots may form in vessels, particularly those serving the limbs or lungs, leading to localised swelling, coolness, or reluctance to bear weight. This pattern reflects the blood's altered clotting balance when protein levels fall.
Marked weight loss despite fluid retention
Muscle and fat stores may diminish even as the abdomen swells, creating a discordant appearance. This reflects the body's struggle to maintain protein reserves whilst continuing to lose them into the urine.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a detailed history—when swelling or other changes first appeared, any recent illnesses or medications, and whether other animals in the household have been affected—followed by a physical examination to assess the distribution of fluid and the general condition of the animal. Blood and urine tests form the foundation of the diagnostic picture, revealing the extent of protein loss and the body's response. Further imaging or tissue sampling may follow when the pattern suggests a specific underlying cause or when initial findings leave the picture incomplete.
Urinalysis and urine protein quantification
Serum biochemistry and protein fractionation
Abdominal imaging
Kidney biopsy and histopathology
Screening for infectious or immune triggers
Options & trade-offs
Management is typically built from a combination of approaches, each addressing a different aspect of the syndrome—protein loss, fluid accumulation, clotting risk, or an identified underlying cause. The combination chosen depends on the severity of signs, the results of investigation, and what the owner and animal can sustain over weeks to months. No single approach reverses nephrotic syndrome in all cases, and the trajectory often unfolds over time as the glomeruli either stabilise, improve, or continue to deteriorate.
Dietary protein modification
Adjusting the protein content and type in the diet can influence the workload on the kidneys and may help reduce the magnitude of protein loss in some animals. Diets formulated for renal support often contain moderate, high-quality protein alongside controlled phosphorus and sodium levels. The approach is typically combined with other interventions rather than used alone.
Trade-offs: Some animals resist the palatability of specialised diets, particularly when appetite is already compromised by low albumin or fluid shifts. The degree of benefit varies, and dietary changes alone rarely halt progressive protein loss when the underlying glomerular damage is severe or immune-mediated.
Immunosuppressive or anti-inflammatory medication
Drugs such as corticosteroids, ciclosporin, or mycophenolate may be used when the nephrotic syndrome is thought to arise from immune-mediated glomerular injury or when biopsy reveals patterns likely to respond to immune modulation. The goal is to reduce inflammation or immune complex deposition within the glomeruli, slowing or halting protein loss. Treatment duration is often prolonged, with gradual dose adjustments based on response.
Trade-offs: Immunosuppressive drugs carry risks of infection, gastrointestinal upset, or metabolic changes, and not all animals respond favourably. In cases where the underlying cause is infectious rather than immune-mediated, suppression of the immune system may worsen the condition, making accurate identification of the trigger important before starting therapy.
Angiotensin-modulating drugs
ACE inhibitors or angiotensin receptor blockers can reduce intraglomerular pressure and may lessen protein leakage through effects on the filtering barrier itself. These drugs are often used regardless of blood pressure levels, as their anti-proteinuric effect can be seen even in animals with normal or low blood pressure. Monitoring kidney function markers and electrolytes during treatment helps identify animals who tolerate the drugs less well.
Trade-offs: Some animals experience worsening kidney function markers or hyperkalaemia when these drugs are introduced, particularly if circulating volume is low or other medications are also affecting electrolyte balance. The reduction in proteinuria is often partial rather than complete, and the drugs do not address the underlying glomerular lesion.
Antithrombotic therapy
Aspirin, clopidogrel, or low-dose heparin may be used to reduce the risk of thrombus formation in animals with severe hypoalbuminaemia, as nephrotic syndrome shifts clotting balance toward hypercoagulability. The decision to start antithrombotic treatment often weighs the invisible risk of clot formation against the visible risk of bleeding, particularly if the animal has concurrent thrombocytopaenia or a recent biopsy.
Trade-offs: There is no universal threshold of albumin or proteinuria at which clotting risk becomes unacceptable, and some animals develop clots despite therapy whilst others tolerate severe hypoalbuminaemia without incident. Monitoring for signs of bleeding or clotting is ongoing, and the drugs do not address the protein loss itself.
Diuretics and fluid management
Furosemide or other diuretics may be used cautiously to mobilise oedema or pleural fluid when swelling interferes with breathing or movement. The goal is to reduce the volume of fluid in tissues without depleting circulating volume so severely that kidney perfusion drops further. Sodium restriction in the diet can reduce the drive to retain fluid, though palatability often limits how strict this restriction can be.
Trade-offs: Overzealous diuresis can worsen kidney function, lower blood pressure, and trigger electrolyte disturbances, particularly in animals already losing protein and fluid regulation. Some animals with severe hypoalbuminaemia respond poorly to diuretics, as the low oncotic pressure in blood vessels makes it difficult to keep mobilised fluid from simply redistributing into other tissue spaces.
Common misconceptions
"Nephrotic syndrome is the same as kidney failure, and once it starts, the kidneys will inevitably stop working altogether."
Nephrotic syndrome describes a pattern of protein loss rather than a specific stage of kidney failure. Some animals maintain stable kidney function for months or years whilst continuing to lose protein, particularly when the underlying cause is identified and managed. Others do progress to chronic kidney disease, but the timeline and outcome depend on the type of glomerular damage, the response to treatment, and whether complications such as clotting or infection intervene.
"Swelling means the kidneys are retaining too much water, so restricting water intake will help reduce the oedema."
The swelling arises because low blood protein allows fluid to leak from vessels into tissues, not because the animal is drinking too much. Restricting water can worsen dehydration and reduce kidney perfusion without addressing the root cause. Fluid management typically involves careful use of diuretics and dietary sodium control rather than limiting access to water.
"A high-protein diet will replace the protein being lost in the urine and restore normal levels in the blood."
Feeding more protein does not reliably raise blood albumin levels when the kidneys are losing large amounts into the urine, and in some cases a very high protein intake may increase the workload on already damaged glomeruli. Diets used in nephrotic syndrome typically focus on moderate, high-quality protein rather than maximum quantity, alongside other adjustments to support kidney function and reduce fluid retention.
Related conditions
Glomerulonephritis
Glomerulonephritis is often the underlying cause of nephrotic syndrome, as inflammation of the glomeruli can damage the filtering units in a way that allows large amounts of protein to leak into the urine. Identifying glomerulonephritis through biopsy or other investigations can help guide decisions about treatment and prognosis.
Protein-Losing Enteropathy
Protein-losing enteropathy can present with similar signs to nephrotic syndrome—particularly swelling and low blood protein—but the protein is lost through the intestines rather than the kidneys. Distinguishing between the two typically involves testing urine for protein loss and sometimes examining stool or intestinal tissue.
Lyme Nephritis
Lyme nephritis is a form of immune-mediated kidney inflammation that can trigger nephrotic syndrome in dogs exposed to Borrelia burgdorferi. The immune response to the infection may damage the glomeruli, leading to significant protein loss and the characteristic features of nephrotic syndrome.
Pleural Effusion
Pleural effusion can develop as a consequence of nephrotic syndrome when the loss of protein lowers oncotic pressure in the bloodstream, allowing fluid to accumulate in the chest cavity. This fluid build-up may contribute to breathing difficulty and is one of several sites where oedema can appear.
Hyperlipidaemia
Hyperlipidaemia often accompanies nephrotic syndrome, as the liver responds to low blood protein levels by increasing production of lipoproteins, which can lead to elevated triglycerides and cholesterol in the bloodstream. This secondary rise in blood fats may be detected on routine testing and tends to improve when the underlying protein loss is addressed.
The pattern of protein loss and fluid shifts in nephrotic syndrome often overlaps with other metabolic and kidney conditions, and exploring how the kidneys regulate fluid, electrolytes, and waste can provide useful context for conversations about long-term monitoring. In some cases, the underlying cause—whether immune-mediated, inherited, or infectious—connects to broader patterns of inflammation or clotting risk that appear elsewhere in the body. Understanding the trajectory of glomerular disease, and the range of ways it can declare itself, can help frame what changes at home might signal a shift worth discussing at the next appointment.