Reading "small left kidney" or "atrophic right kidney" on an ultrasound report can be worrying. This guide explains what a small kidney and kidney atrophy mean, how a kidney born small differs from one that shrank, the main causes and tests, who follows it, how to protect kidney function and which signs need prompt attention.
What does a small kidney mean?
A small kidney is a kidney that measures smaller than expected on a scan. According to the National Kidney Foundation, an adult kidney is usually about 10 to 12 cm long, roughly the size of a fist. A small kidney can affect one side or both, and is also called kidney atrophy, an atrophic kidney or renal atrophy.
During a urinary tract ultrasound, the sonographer measures the length of each kidney and the thickness of its tissue (the cortex) and compares the two sides. Length alone is not enough: the doctor also considers age, body size, shape and function.
Born small or shrunk later?
| Feature | Hypoplasia (congenital) | Atrophy (acquired) |
|---|---|---|
| When it starts | From birth | After the kidney was a normal size |
| Appearance | Small but usually regular in structure | May show scars or thinned tissue |
| Cause | Fewer filtering units from development | Disease, blockage, infection or poor blood supply |
| Nuclear (DMSA) scan | Regular outline, even uptake | Outline defects, uneven uptake |
According to a 2020 study in the journal Renal Failure, a hypoplastic kidney is small with normal structure but fewer nephrons, while atrophy results from scarring after infection or obstruction in a kidney that was a normal size at birth.
Causes of kidney atrophy
According to the National Kidney Foundation, common reasons a kidney shrinks include:
- Renal artery stenosis: narrowing of the artery that supplies the kidney reduces its blood flow. According to the NIDDK, about 90 percent of cases are caused by atherosclerosis.
- Long-standing urinary blockage: back-pressure damages the filtering units; an untreated kidney stone can be the cause.
- Repeated or long-lasting kidney infections: these can leave scars. The American Urological Association links urine reflux from the bladder to the kidney, combined with kidney infection, to a higher chance of scarring in children.
- Autoimmune and other long-term kidney diseases: when both kidneys are affected they may shrink over time; according to the National Kidney Foundation, atrophy of both kidneys can lead to chronic kidney disease.
Does a small kidney cause symptoms?
Often it causes none, especially when it develops slowly, and is found by chance on a scan. According to the National Kidney Foundation, symptoms, when present, usually relate to the cause or to reduced function: flank or abdominal pain, pain when passing urine, blood in the urine, frequent urination, tiredness, poor appetite, itchy skin and swelling of the hands and feet. Renal artery stenosis may show up as high blood pressure that is hard to control.
How the other kidney compensates
If only one kidney is small and the other is healthy, the healthy kidney usually takes on most of the work. The National Kidney Foundation states that one healthy kidney can work as well as two, and that when only one kidney is affected there may be few or no health problems.
According to a 2020 review in Frontiers in Physiology, the remaining kidney grows and each of its nephrons filters more. This restores overall function but may, over many years, contribute to high blood pressure or kidney strain. So regular check-ups matter even without symptoms. For more, see living with one kidney.
How a small kidney is diagnosed
- Ultrasound: usually the first test; it shows the size and shape of the kidneys and any scarring or blockage. The National Kidney Foundation notes that CT or MRI may also be used.
- Blood tests: creatinine and estimated glomerular filtration rate (eGFR) show how well the kidneys filter. Read about kidney function tests.
- Urine tests: the urine albumin-to-creatinine ratio (uACR) checks for protein leakage, and other tests look for infection.
- DMSA nuclear scan: this scan shows scarring and how much each kidney contributes. In children with urine reflux, the American Urological Association recommends it when the ultrasound is abnormal or scarring is a particular concern; in other cases the doctor decides whether it is needed.
- Blood vessel imaging: according to the NIDDK, renal artery stenosis can be assessed with duplex ultrasound, CT angiography, MR angiography or a catheter angiogram.
Who follows it, and protecting kidney function
A urologist assesses structural causes such as blockage, stones and reflux, while a nephrologist (kidney medicine specialist) manages reduced function, blood pressure and protein in the urine over the long term. They often work together, with the urologist referring when needed. See nephrologist or urologist for the difference.
If the small kidney contributes little, you effectively have one working kidney; according to the National Kidney Foundation, people with one working kidney should have their kidney function and blood pressure checked at least once a year. General steps to protect your kidneys include:
- Keeping blood pressure and diabetes under control and checked regularly.
- Avoiding over-the-counter anti-inflammatory painkillers (NSAIDs) unless a doctor advises them, as the National Kidney Foundation recommends.
- Telling doctors and pharmacists about your kidney before new medicines or contrast scans.
- Cutting down on salt, not smoking and keeping a healthy weight.
Treatment depends on the cause and is decided by your doctor. According to the National Kidney Foundation, a non-working small kidney usually does not need removing if the other kidney works fully, unless it causes an ongoing problem such as repeated infections.
When to see a doctor urgently
Seek urgent care or see a doctor straight away if you have:
- High fever with shivering and flank pain, which may signal a kidney infection.
- Severe flank pain, or passing very little or no urine.
- Visible blood in the urine.
- Very high blood pressure with headache or vision changes.
- Sudden swelling of the legs or face, breathlessness or unusual severe tiredness.
If a small kidney was found by chance and you feel well, book a routine appointment.
Frequently asked questions
Is a small kidney dangerous?
It depends on the cause and on the other kidney. With one small kidney and a healthy other kidney, there may be few or no problems. Two small kidneys can point to chronic kidney disease, which needs regular follow-up with a kidney specialist.
Can an atrophic kidney grow back to normal size?
Scarred kidney tissue usually does not recover. Care focuses on finding and stopping the cause, such as relieving a blockage, treating infection or controlling blood pressure, and on protecting remaining function. Your doctor plans this after testing.
Does a small kidney need to be removed?
Usually not. According to the National Kidney Foundation, a non-working kidney is generally left in place if the other kidney works fully, unless it causes an ongoing problem such as repeated infections. The decision rests with your treating doctor after a full assessment.
What is the difference between hypoplasia and atrophy?
Hypoplasia means the kidney was small from birth and has fewer filtering units, but its structure is usually regular. Atrophy happens in a kidney that was normal and then shrank because of disease, blockage or infection, and scans may show scarring.
Does a small kidney affect pregnancy?
A woman with one working kidney may need closer monitoring in pregnancy; the NIDDK lists preeclampsia among possible but uncommon problems with a single-functioning kidney. Tell your obstetrician about the kidney so blood pressure and kidney function can be checked.
How often should kidney function be checked?
If the small kidney contributes little, you effectively have one working kidney; according to the National Kidney Foundation, people with one working kidney should have blood and urine kidney tests and a blood pressure check at least once a year. Your doctor may test more often if there are signs that kidney function is falling.
Sources
- Kidney Atrophy – Symptoms, causes, treatment | National Kidney Foundation
- Living With One Kidney | National Kidney Foundation
- Solitary or Single-functioning Kidney – NIDDK
- Renal Artery Stenosis – NIDDK
- The comparison of the resistivity index values in the ultrasonographic evaluation of a unilateral atrophic/hypoplastic kidney (Renal Failure, 2020)
- Vesicoureteral Reflux Topics – American Urological Association
- Physiology and Pathophysiology of Compensatory Adaptations of a Solitary Functioning Kidney (Frontiers in Physiology, 2020)
Book a consultation
Prof. Al-Samarrai and Dr. Amer Mousa assess kidney and urinary problems at Prof. Al-Samarrai Medical Center, Dubai Healthcare City, discuss the options that suit your case, and refer you to a nephrologist when needed. Medical consultations are confidential.
Read more: Urologist in Dubai: doctors, services, insurance and booking
This information is for health education and does not replace a medical examination. See our medical content policy.