Cat Stress Hormone Axis: Cortisol and the Clock

A cat's cortisol can double between the parking lot and the exam table, and it keeps climbing through the first 15 minutes of restraint. That single fact decides how a stress hormone result should be read: the number only means something next to the clock time, the order of procedures, and how long the cat had already been in the building. Most confusing adrenal results in cats are sampling problems before they are adrenal problems.

Four numbers frame the axis: 15 minutes to a stress peak, 60 to 90 minutes for cortisol to fall by half, 3 minutes to finish the draw once the needle is in, and 20 to 30 minutes of quiet if that window is missed.

The loop in one pass

The hypothalamus releases corticotropin-releasing hormone (CRH) in pulses, roughly 6 to 12 bursts an hour in a resting cat. Each burst tells the pituitary to release ACTH, and ACTH tells the adrenal cortex to release cortisol. Circulating cortisol then feeds back on both the hypothalamus and the pituitary, cutting CRH and ACTH output within 10 to 15 minutes. A healthy cat sits near a 24-hour mean of 1 to 3 µg/dL (28 to 83 nmol/L), with wide swings around that mean.

Two feedback speeds matter. The fast arm, cortisol shutting down ACTH release, works in 10 to 15 minutes and is exactly what makes a 30-minute protocol possible. The slow arm, which changes how much CRH the hypothalamus can manufacture, takes hours to days. An acute stressor shows up as one high cortisol. Chronic stress more often shows up as a flattened daily curve than as a high single value.

Where the draw time lands on that curve

Cats are less sharply diurnal than dogs, but most laboratories still expect a fixed collection hour, because values in the same cat drift 2 to 3 fold across a day. Common practice is a draw between 08:00 and 10:00, repeated at the same clock time if it comes back abnormal. A 16:00 sample cannot be read against an interval built from morning cats, and repeating it next week at a different hour repeats the error rather than correcting it.

Procedure order matters as much as the hour. A cortisol collected after a full physical, a rectal temperature, and 2 failed venipuncture attempts is a stress measurement wearing a baseline label. The workable sequence is to weigh and watch first, draw within 3 minutes of entering the room, then do the hands-on exam. If the first attempt fails, stop, sit the cat in a quiet carrier for 20 to 30 minutes, and read the second sample as a 20-minute post-stress value.

Why ACTH is the fragile one

Cortisol is forgiving. Serum or plasma spun within 2 hours holds for 24 to 48 hours at 4 C and for months at -20 C. ACTH is a 39-amino-acid peptide that adsorbs to glass and is chewed up by blood enzymes, so it needs an EDTA tube, immediate chilling, separation within 15 minutes, and freezing at -20 C or colder within 30 to 60 minutes. A sample that sat on the bench for 2 hours can read 30 to 50% low and imitate pituitary disease that is not there.

Ranges are lab-specific, timing is not

The intervals below are the shapes most reference laboratories report for cats; the cut-offs in your own report may move by 20 to 30% and are the ones to use. What travels between labs is the timing rule, not the number.

Analyte	Tube	Spin within	Stable at 4 C	Typical interval
Cortisol, morning baseline	serum or EDTA plasma	120 min	24-48 h	1-4 µg/dL (28-110 nmol/L)
Cortisol, evening	serum or EDTA plasma	120 min	24-48 h	0.5-3 µg/dL (14-83 nmol/L)
Cortisol, 60 min after stimulation	serum or EDTA plasma	120 min	24-48 h	6-12 µg/dL (165-330 nmol/L)
ACTH	EDTA, chilled	15 min	4-8 h	10-60 pg/mL
Urine cortisol to creatinine	free catch	same day	24 h, dark	usually under 10 x 10-6

Stress that is not adrenal disease

A 20-minute car ride lifts cortisol 1.5 to 2 fold. Three strange cats in a waiting room add more. A room at 28 C, a barking dog in the corridor, and 5 minutes of lateral restraint can push a cat from 2 µg/dL to 8 or 9 µg/dL without any adrenal lesion. Hospitalised cats sampled on day 1 and again on day 3 often give two different answers for that reason alone.

Take the sample before the stress, or account for it afterwards. When a result is borderline, the cheapest next step is not another test but another draw on a different day, at the same hour, with the cat admitted straight into a quiet room and left for 20 minutes before anyone touches it.

When the number does mean disease

Red flags are a fasting cortisol above 10 µg/dL with heavy drinking and urination, a pot belly, thinning skin and muscle wasting over the spine; or the opposite pattern, a low baseline with an ACTH that will not rise after stimulation and a cat that is weak, vomiting and off food. Those cats need a full workup, not a repeat in 6 months. Weakness, vomiting, collapse or a cat that stops eating for 24 hours is a same-day visit.

One value is also not a diagnosis in the other direction: a single low cortisol in a sick cat is common and usually reflects the illness, not a failing adrenal gland. Pair it with ACTH and with the clinical picture before anyone labels the cat.

Six steps to a sample worth trusting

Book the draw between 08:00 and 10:00, and write the clock time on the form next to the result.
Let the cat sit in a quiet carrier 20 minutes after arrival; no exam, no weighing, no other animals in sight.
Draw within 3 minutes of handling; if it fails, wait 20 to 30 minutes rather than trying a third vein.
For ACTH use a chilled EDTA tube, spin within 15 minutes, freeze within 60 minutes.
Repeat any abnormal cortisol at the same hour on a different day before treating.
Send the same analyte to the same laboratory: a 20 to 30% assay shift between labs looks like a trend.

The same clock logic applies to the thyroid, where the feedback loop runs slower but the interference list is longer; see the thyroid hormone axis. If you are unsure how old the cat is and whether a kitten curve applies at all, the breed and age quiz narrows the range.

Frequently asked

Does one high cortisol mean my cat has Cushing's disease?

Usually not. A single elevated value after a car ride, a wait in a busy room and 5 minutes of restraint is common in healthy cats. Ask for the draw time, whether the sample came before or after the exam, and a repeat at the same hour on another day. Persistent values above 10 µg/dL together with heavy drinking, a pot belly and muscle loss over the spine are what push the workup forward.

Why does the vet want the blood drawn so fast?

Because cortisol starts rising within 2 to 4 minutes of handling and is clearly elevated by 15 minutes. A draw finished inside 3 minutes still reflects the resting state; a draw that takes 8 minutes with two failed attempts reflects the struggle. If the first attempt fails, the honest option is to wait 20 to 30 minutes in a quiet carrier and label the result as a post-stress sample.

Can I just repeat the test at home or at another clinic?

A different clinic is fine, but keep the collection hour and, where possible, the same laboratory, because assay methods differ by 20 to 30% and that gap is easy to misread as a trend. Note the clock time on the request form every time. If your cat is weak, vomiting, collapsed or has not eaten for 24 hours, skip the repeat and go in today.