Nursing care
Cortisol and ACTH Testing: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
Cortisol and ACTH must be interpreted against the time of the draw, since cortisol peaks around 0600-0800 and troughs at night. A high morning cortisol that fails to suppress with dexamethasone points to Cushing syndrome. A cortisol that fails to rise after ACTH stimulation points to Addison disease and adrenal insufficiency.
Why this value is ordered
Cortisol and ACTH are ordered when a patient has signs that don't fit anywhere else: unexplained weight change, fatigue that doesn't lift, hyperpigmentation, purple striae, or a blood pressure that won't behave. The two hormones are read together because ACTH from the pituitary drives cortisol from the adrenal cortex, so a problem in either gland changes both numbers, and the pattern between them tells you which gland is failing.
The workup usually starts with a baseline cortisol drawn at a set time, then moves to a dynamic test if the baseline is ambiguous. The dexamethasone suppression test asks whether the adrenal glands can be switched off. The ACTH stimulation test asks whether they can be switched on. Ordering one instead of the other depends on whether the clinician suspects excess cortisol or a deficiency.
Interpreting the number in context
Cortisol follows a diurnal rhythm: it peaks in the early morning and drops to its lowest point around midnight. A cortisol drawn at 8 a.m. and one drawn at 4 p.m. are not comparable, and a single number without a documented draw time is close to useless. Always check the requisition for the collection time before you act on the result.
In the dexamethasone suppression test, the patient takes dexamethasone the night before and cortisol is drawn the next morning. A normal adrenal system suppresses cortisol production in response; failure to suppress supports Cushing syndrome. In the ACTH stimulation test, synthetic ACTH (cosyntropin) is given and cortisol is measured before and after. A normal adrenal gland responds with a sharp rise; a blunted or absent rise supports Addison disease. Keep the two tests straight: suppression rules Cushing in, stimulation rules Addison in.
Critical values and what to do
A very low cortisol in a patient who is hypotensive, vomiting, or acutely unwell is an adrenal crisis until proven otherwise. This is a call, not a note in the chart. The patient needs IV hydrocortisone and IV fluids before the confirmatory labs come back, because waiting for results can be fatal in true adrenal crisis.
A markedly elevated cortisol with a suppressed ACTH points to an adrenal source, such as a cortisol-secreting adrenal tumor. A markedly elevated cortisol with a high ACTH points to a pituitary or ectopic source. Either pattern needs endocrinology involvement, but the adrenal crisis scenario is the one where minutes matter.
Related tests read alongside it
A 24-hour urine free cortisol or late-night salivary cortisol often accompanies the blood work, since a single serum value can miss the diurnal swing that urine and saliva testing capture over time. Electrolytes matter too: Addison disease classically produces hyponatremia and hyperkalemia from aldosterone loss, while Cushing syndrome can produce hypokalemia and metabolic alkalosis.
Glucose tracks with cortisol in both directions. Cushing syndrome tends to push glucose up, sometimes to the point of new-onset diabetes, while Addison disease can cause fasting hypoglycemia. A CT or MRI of the adrenal glands or pituitary usually follows an abnormal hormone result to locate the source.
Nursing implications
Confirm the draw time before the sample leaves the unit, and document it on the label if your system allows free text. A morning cortisol drawn at noon by mistake will read falsely low and can send the workup in the wrong direction. If dexamethasone was given the night before a suppression test, confirm the patient actually took it and note the time.
Watch for signs of adrenal crisis in any patient with known or suspected adrenal insufficiency who is under physiologic stress from surgery, infection, or trauma. These patients often need stress-dose steroids before a procedure, and missing that step is a preventable emergency. Teach patients on chronic steroid therapy never to stop it abruptly, since the adrenal glands may be too suppressed to compensate.
What patients ask about it
Patients often ask why the blood draw has to happen so early, and the honest answer is that the result is meaningless at any other time without a different reference range. Explain that cortisol is highest around waking and lowest around midnight, so the lab schedules the draw to catch the peak.
Patients on the suppression test often ask why they're taking a steroid pill the night before a blood test. Explain that the dose is small and the point is to see whether their own body correctly shuts off cortisol production in response, not to treat anything. Patients facing an ACTH stimulation test sometimes worry about the injection itself; reassure them it is a single dose of synthetic hormone and the blood draws before and after are what matter.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our endocrine practice questions are the closest set to what this page covers.
Common questions
Why is cortisol drawn in the morning instead of any time?
Cortisol follows a diurnal rhythm and peaks between roughly 6 and 8 a.m., then falls through the day to its lowest point near midnight. A result drawn outside that window is compared against the wrong reference range and can mislead the whole workup, so the collection time has to be documented and correct.
How do I remember which test goes with which condition?
Suppression tests for Cushing, stimulation tests for Addison. Dexamethasone suppression asks whether excess cortisol can be turned off, which is the Cushing question. ACTH stimulation asks whether cortisol can be turned on, which is the Addison question.
What does a low ACTH with a low cortisol mean?
This pattern points to secondary adrenal insufficiency, where the pituitary isn't producing enough ACTH to drive the adrenal glands, rather than primary adrenal failure. Common causes include pituitary disease or abrupt withdrawal from long-term steroid therapy.
What's the priority nursing action for suspected adrenal crisis?
Notify the provider immediately and prepare for IV hydrocortisone and aggressive fluid resuscitation; don't wait for confirmatory labs. Monitor for hypotension, hyponatremia, hyperkalemia, and hypoglycemia while treatment starts.