Urine Specific Gravity in Cats: Normal Range & What Low USG Means

Urine specific gravity is one of the most important numbers on a cat urinalysis — and cats have a critically different normal range than dogs. Because cats evolved as desert hunters who rarely drank water, a healthy cat's kidneys concentrate urine far more than a dog's. That makes even modest decreases in USG a meaningful early warning sign.

Why Cats Have Higher USG Than Dogs

Cats are obligate carnivores that evolved in arid environments. They have longer renal tubular loops of Henle than dogs, creating a steeper concentration gradient in the kidney medulla. This allows cats to produce urine up to four times as concentrated as their blood plasma — something dogs cannot do.

This biology has two important clinical implications:

• • A healthy cat should have USG ≥ 1.035. Anything below this is already inadequate for the species.
• • A cat with USG 1.025 may look "above isosthenuria" on paper but has lost significant concentrating ability compared to its healthy baseline. Always trend USG over time.

USG is measured with a refractometer. The dipstick pad for specific gravity is unreliable in cats and should not be used for clinical decisions.

USG Interpretation Chart for Cats

The Hyperthyroidism–CKD Connection: A Critical Cat-Specific Trap

In cats, hyperthyroidism and CKD are both extremely common in senior cats — and they mask each other in a way that directly affects how you interpret USG.

High thyroid hormone increases cardiac output and raises kidney blood flow (GFR), making BUN and creatinine look better than they really are. A hyperthyroid cat with underlying CKD may have perfectly normal creatinine — but its USG will already be declining. USG drops before bloodwork does.

When methimazole or radioiodine normalizes thyroid levels, GFR drops back to its true level. Creatinine and BUN rise — sometimes dramatically — revealing CKD that was always there but hidden. This is not treatment failure; it's unmasking. Your vet should recheck kidney values 2–4 weeks after starting hyperthyroid treatment. Some cats are deliberately kept on lower thyroid doses to avoid worsening renal function.

Causes of Low USG in Cats

• • Chronic kidney disease (CKD) — the most common cause in cats over 7. As nephrons are lost the medullary gradient flattens; concentrating ability often falls before BUN and creatinine rise. See SDMA in cats.
• • Hyperthyroidism — high GFR produces dilute urine (USG 1.020–1.035). Dilute urine + elevated T4 + normal creatinine should prompt evaluation for masked CKD before starting treatment.
• • Diabetes mellitus — glucosuria acts as an osmotic diuretic, diluting the urine. Typical USG 1.018–1.030 with glucose on dipstick.
• • Diabetes insipidus — rare in cats but causes extreme hyposthenuria below 1.007 with enormous urine volumes. Central (no ADH) or nephrogenic (resistance to ADH).
• • Hypercalcemia — from idiopathic hypercalcemia, primary hyperparathyroidism, or lymphoma. High calcium impairs tubular ADH response and can cause permanent calcium nephropathy.
• • Acromegaly — pituitary IGF-1 excess causes insulin-resistant diabetes and glucosuria. Cats with acromegaly typically have very dilute urine alongside extreme insulin requirements.
• • Liver disease / hepatic lipidosis — severe hepatic insufficiency reduces urea production, undermining the kidney's concentration gradient.

FLUTD and USG: Why Cats With Bladder Disease Often Have High USG

Cats with feline idiopathic cystitis (FIC) — the most common form of FLUTD — typically have highly concentrated urine (USG >1.040), not dilute urine. Concentrated, mineral-rich urine irritates the bladder wall and promotes struvite crystal formation.

If your cat has signs of FLUTD (straining, blood in urine, frequent trips to the litter box) and the USG is actually low, that's a red flag. Don't assume idiopathic cystitis — investigate for metabolic disease. Conversely, increasing water intake to dilute the urine (via wet food, water fountains) is a core part of managing recurrent FIC.

USG and CKD: Why Serial Monitoring Matters in Cats

Because cats normally have such high USG, a gradual decline — even if still technically "above isosthenuria" — is a meaningful early signal. Consider these two trajectories:

CKD affects 30%+ of cats over 15 years. BUN and creatinine don't rise until 66–75% of nephron function is lost — but USG decline begins much earlier. Running a urinalysis alongside annual bloodwork creates the trend data that allows early intervention — dietary phosphorus restriction and targeted hydration — before the disease advances.

What Your Vet Will Do Next

Symptoms That May Accompany Low USG in Cats

Cats with persistently dilute urine may show:

• • Drinking more water than usual (polydipsia) and urinating more
• • Weight loss, especially muscle mass over the spine and hips
• • Decreased appetite or not eating
• • Vomiting (uremic nausea in CKD)
• • Unkempt coat, reduced grooming
• • Pale, nearly colorless urine

Frequently Asked Questions