Adrenal Disease in Senior Cats: Cushing's vs Addison's

If a diabetic cat still runs above 300 mg/dL on more than 1.5 units of insulin per kilogram per dose, and its skin splits when you lift it under the front legs, the insulin is not the problem. In cats, 80 to 90% of Cushing's syndrome cases arrive with diabetes already diagnosed, and the median gap between the two diagnoses is 4 months. The adrenal gland is the second patient.

Two diseases, one gland

The adrenal cortex fails in two opposite directions. Hyperadrenocorticism (HAC, Cushing's syndrome) means too much cortisol. Hypoadrenocorticism (Addison's disease) means too little cortisol, too little aldosterone, or both. Both are rare in cats, and both are routinely mistaken for something common.

Among cats with HAC, 75 to 85% have a pituitary tumour driving both adrenal glands, and 20 to 25% have a cortisol-secreting adrenal tumour. Median age at diagnosis in a 30-cat multicentre series was 13.0 years (range 4.0 to 17.6); a review of more than 180 published cases put the mean at 10 years. Addison's is rarer: 48 cases documented between 1983 and 2023, median age 4 years. That is a young cat's disease.

Why Cushing's hides inside a diabetes diagnosis

Cortisol opposes insulin. A cat with untreated HAC needs more insulin for the same glucose, so the first sign is a dose that keeps climbing. Insulin resistance is generally suspected above 1.5 units/kg per dose, or when glucose will not fall below 300 mg/dL during a curve; fructosamine in resistant cats usually sits above 500 µmol/L and can exceed 700.

The trap points the other way too: most cats with poorly controlled diabetes do not have HAC. In that 30-cat series, 27 had diabetes, and the interval from diabetes diagnosis to HAC diagnosis ran 0 to 24 months. Test when the dose is high and the skin or body shape has changed, not on dose alone.

Skin and body changes worth photographing

Dermatologic abnormalities appeared in all 30 cats: thin skin in 70%, hair loss in 60%, skin lacerations in 57%. Abdominal distension and muscle wasting each showed in 67%, polyuria and polydipsia in 87%, polyphagia in 70%.

Skin fragility is the finding that changes handling. It is reported in up to a third of cases, and in about 15% in one dermatology teaching text. Skin can tear during lifting, grooming, or from a blood pressure cuff or adhesive tape. Use a towel, skip tape, and warn the clinic before restraint.

Two blood-work habits send owners down wrong roads. Unlike dogs, cats with HAC usually have normal alkaline phosphatase, and only about half show a stress leukogram. Four of 25 cats also had low total T4 — sick-euthyroid physiology, not hypothyroidism.

Test order: dexamethasone first, ACTH second

The low-dose dexamethasone suppression test (LDDST) is the screening test in cats, at ten times the canine dose: 0.1 mg/kg IV, with cortisol measured before, at 4 hours and at 8 hours. Failure to suppress, meaning cortisol above 1.4 µg/dL at either point, is consistent with HAC; AAHA's interpretive guide calls 1.1 to 1.4 µg/dL inconclusive. Sensitivity in the 30-cat series was 96% (27 of 28).

The ACTH stimulation test screens badly in cats. It flagged only 9 of 16 cats (56%), and was normal in all three cats with an adrenal tumour. Its real job is monitoring trilostane. A urine cortisol:creatinine ratio can pre-screen, from two morning samples collected at home: above 20 is supportive, below 20 makes HAC very unlikely, but a high ratio still needs an LDDST.

Pituitary or adrenal tumour

The answer decides whether surgery can cure the cat. Endogenous ACTH below 10 pg/mL points to an adrenal tumour; above 12 pg/mL points to pituitary disease. Adrenal ultrasound matched the final diagnosis in 28 of 30 cats (93%), and a high-dose dexamethasone test at 1.0 mg/kg or cross-sectional imaging settles the remainder.

Treatment: trilostane, surgery, radiation

Trilostane, a steroid-synthesis inhibitor, is the medical mainstay at roughly 1 mg/kg given two to three times daily with food. That dose usually means a compounded capsule, because no trilostane product is licensed for cats in the US. In 15 cats, clinical signs improved in 13, and median survival was 617 days (range 80 to 1,278); in 6 of 9 diabetic cats the insulin dose fell by 36% within two months. Monitoring is by ACTH stimulation plus electrolytes, because hypocortisolemia is the failure mode.

A unilateral adrenal tumour is best treated by adrenalectomy, which can be curative. Pituitary-dependent disease is managed with trilostane, pituitary radiation, or hypophysectomy where available. Mitotane has performed poorly in cats.

Addison's disease: the crash that mimics kidney failure

Forty-eight documented cases are the entire evidence base, so patterns matter more than any single figure. Median age 4 years. Anorexia and lethargy each in 25 of 48 cats, vomiting and weight loss in 14, dehydration in 24. Azotemia appeared in 30 of 48, which is why these cats get labelled as kidney cases. Azotemia that clears after 24 hours of fluids is the tell.

Electrolytes are less reliable than in dogs. A sodium:potassium ratio below 27 was documented in only 12 of 48 cats, and was normal in 10. The diagnosis is the ACTH stimulation test: a resting cortisol at or below 2 µg/dL, then a post-ACTH cortisol under 2 µg/dL. A 2025 series of 41 cats used a cut-off under 3 µg/dL (83 nmol/L).

Long-term treatment pairs prednisolone 2.5 to 5 mg daily with a mineralocorticoid: DOCP at 2.2 mg/kg every 25 days, with intervals running 21 to 38 days, or fludrocortisone at 0.01 mg/kg twice daily. Neither is licensed for cats. In a crisis, 10% calcium gluconate at 0.5 mL/kg IV protects the heart while potassium is corrected. Of 39 cats with follow-up, 7 were euthanased; the longest reported survival is 70 months.

What the tests and drugs cost

Item	Figure	What it covers
Cortisol, single sample	$32	US university lab fee, per tube
LDDST	3 cortisol samples	Baseline, 4 hours, 8 hours
ACTH stimulation	$62	US university lab fee, plus clinic time
Cortisol panel, UK lab	£41	Three samples for an ACTH test
Trilostane, 30 capsules	$35 to $160	Compounded, dose-dependent

Ask which test confirms the diagnosis in this cat, and why that one.
Ask for the 30-day monitoring schedule in writing: ACTH stimulation, electrolytes, glucose curve.
Ask what happens to the insulin dose once cortisol falls — it can drop within weeks.
Ask whether the adrenal glands have been imaged, because one tumour is surgically curable.

This article does not replace veterinary diagnosis. Adrenal testing and every drug dose above belong to a veterinarian who has examined the cat; trilostane, DOCP and fludrocortisone are all used off-label in cats.

Terms mentioned in this article

Frequently asked

My diabetic cat needs 3 units twice a day and still runs high. Is that Cushing's?

Possibly, but dose alone proves nothing. Resistance is usually suspected above 1.5 units/kg per dose, so a 4 kg cat would be around 6 units per injection. Most poorly regulated diabetics have no adrenal tumour or pituitary tumour; diet, injection technique, insulin type and infection are more common causes. What raises the odds of Cushing's is a high dose plus thin skin, a pendulous abdomen, muscle wasting, or hair that will not regrow after clipping. The next step is a low-dose dexamethasone suppression test at 0.1 mg/kg, not a higher insulin dose.

Does a normal ACTH stimulation test rule out Cushing's in a cat?

No. In a 30-cat series the ACTH stimulation test detected only 56% of cats with confirmed disease, and it was normal in all three cats that turned out to have an adrenal tumour. The screening test in cats is the low-dose dexamethasone suppression test, which caught 27 of 28 in the same series. The ACTH stimulation test is still useful after treatment starts, to check that trilostane is not pushing cortisol too low.

Can Addison's disease in a cat have normal electrolytes?

Yes, and that is the main reason it is missed. In 48 documented cases, a sodium:potassium ratio below 27 appeared in only 12, and was normal in 10. Azotemia was far more common, at 30 of 48, so many of these cats are treated for kidney disease first. A cat whose azotemia resolves within 24 hours of fluid therapy, or that improves repeatedly on steroids and then relapses, needs an ACTH stimulation test regardless of its electrolyte values.