ACTH Stimulation Testing: Why 18 Is Not a Universal Pass/Fail Number

ACTH Stimulation Testing: Why 18 Is Not a Universal Pass/Fail Number

What if the cortisol number you’ve been told determines whether your adrenal glands are functioning properly isn’t actually the right number for the test your laboratory used?

For decades, people undergoing ACTH stimulation testing have heard the same rule: Your cortisol needs to reach 18 µg/dL to pass. But advances in laboratory testing have revealed that interpreting adrenal function is far more complicated than comparing a result to one universal cutoff.

In this episode of the My Adrenal Life Podcast, we explore new research challenging the traditional 18 µg/dL threshold and explain why the laboratory assay, testing protocol, sample timing, medication history, and type of adrenal insufficiency all influence what a cortisol result actually means.

We begin with an October 2026 systematic review examining the Roche Elecsys Cortisol II assay. After analyzing six studies involving 430 adults and children, researchers proposed a peak stimulated cortisol cutoff of approximately 14.7 µg/dL for the standard 250-microgram ACTH stimulation test.

That’s considerably lower than the traditional 18 µg/dL threshold.

But does that mean someone who previously failed an ACTH stimulation test might now pass?

Possibly—but only when the laboratory method, testing protocol, and clinical circumstances support that interpretation.

We explain why newer cortisol assays often report lower concentrations than older testing methods, even when the patient’s underlying adrenal function hasn’t changed. The biology may be the same, but the measuring stick is different.

We also break down what an ACTH stimulation test actually measures: the adrenal glands’ ability to produce cortisol when stimulated by synthetic ACTH. This distinction is especially important when comparing primary adrenal insufficiency with secondary, tertiary, and steroid-induced adrenal insufficiency, where impaired pituitary signaling, adrenal responsiveness, and the timing of HPA-axis suppression can complicate results.

Other important questions we explore include:

• Why do 30-minute and 60-minute cortisol results matter? Cortisol may peak at different times, and a cutoff must match the protocol used.
• Is a morning cortisol test the same as an ACTH stimulation test? No. These tests answer different questions, particularly when evaluating recovery after glucocorticoid exposure.
• Can estrogen and cortisol-binding proteins change test results? Yes. Changes in binding proteins can influence total measured cortisol without producing equivalent changes in free cortisol.
• How do previous steroid treatments affect testing? Oral medications, injections, inhaled glucocorticoids, and other exposures can influence HPA-axis function and, in some circumstances, laboratory measurements.
• What does a passing result actually mean? A reassuring stimulated cortisol result does not necessarily establish that every aspect of the body’s natural stress response has recovered.

We also discuss the 2024 Endocrine Society and European Society of Endocrinology guidance on evaluating recovery from glucocorticoid-induced adrenal insufficiency, including why morning cortisol is interpreted differently from stimulated cortisol.

Perhaps most importantly, we explore what patients should ask their endocrinologists when reviewing results—especially when those results may influence decisions about glucocorticoid replacement, HPA-axis recovery, or ongoing sick-day and emergency precautions.

The new 14.7 µg/dL proposal does not mean everyone below 18 is suddenly normal. And it does not mean everyone above 14.7 has a fully functioning cortisol stress response.

To learn more, explore the My Adrenal Life Books Authored by founder Rick Disler available on Amazon, visit www.myadrenallife.com, and join our supportive My Adrenal Life Facebook Group for education, discussion, and community.