Testing for adrenal insufficiency typically starts with a simple morning blood draw for cortisol, then moves to a stimulation test if the result falls in an uncertain range. The process sounds straightforward, but the details matter enormously: which cortisol cutoff your lab uses, whether you’re on oral contraceptives, what time you actually woke up, and whether your doctor suspects the problem lies in the adrenal glands themselves or in the pituitary gland that controls them. Each of these factors changes which test is appropriate and how to read its results.
Morning Cortisol as the Starting Point
Cortisol follows a strong daily rhythm, peaking shortly after waking and declining through the afternoon and evening. A blood sample drawn between about 8 and 9 a.m. captures that peak and serves as the initial screen for adrenal insufficiency. The traditional teaching is that a morning cortisol below roughly 100 nmol/L (about 3.6 µg/dL) strongly suggests adrenal insufficiency, while a value above 500 nmol/L (about 18 µg/dL) usually rules it out, making further testing unnecessary.
Recent work, however, suggests those upper thresholds may be too conservative, sending many people for stimulation tests they don’t actually need. One study found that a morning cortisol at or above 362 nmol/L (roughly 13 µg/dL) caught every case of adrenal insufficiency with perfect sensitivity in their population, meaning almost a fifth of patients could have been spared confirmatory testing compared to the traditional 500 nmol/L cutoff.1Endocrine and Metabolic Science. Predictive value of different thresholds of morning cortisol in diagnosing adrenal insufficiency Another group using a newer-generation cortisol assay found that even lower morning cortisol values could reliably predict or exclude adrenal insufficiency, depending on which stimulation-test cutoff was used as the reference standard.2PubMed Central. Morning cortisol and central adrenal insufficiency: new thresholds from low-dose ACTH test and second-generation assay
The practical upshot: if your morning cortisol is very low, the diagnosis is likely and your doctor can often move straight to treatment and cause-finding. If it is clearly high, you probably don’t have adrenal insufficiency. The tricky zone in between, which includes a large share of people tested, requires a stimulation test to settle the question.
The ACTH Stimulation Test
The cosyntropin (Synacthen) stimulation test is the most commonly used confirmatory test worldwide. Cosyntropin is a synthetic fragment of ACTH, the pituitary hormone that tells the adrenals to produce cortisol. You receive an injection, and blood is drawn 30 and sometimes 60 minutes later. If the adrenals are healthy and receiving adequate ACTH signaling over time, they should respond with a robust cortisol rise.
There are two versions. The standard high-dose test uses 250 µg of cosyntropin, a pharmacological dose far exceeding what the pituitary would ever release naturally. The low-dose test uses just 1 µg, which is closer to a physiological stimulus. A systematic review and meta-analysis found that the two doses have similar overall diagnostic accuracy.3The Journal of Clinical Endocrinology & Metabolism. ACTH Stimulation Tests for the Diagnosis of Adrenal Insufficiency: Systematic Review and Meta-Analysis Where they differ is in detecting milder or more recent adrenal suppression. In secondary adrenal insufficiency, where the problem is insufficient ACTH from the pituitary rather than damage to the adrenal glands themselves, the massive dose in the high-dose test can sometimes force a normal-looking cortisol response from adrenals that have only been understimulated for a short time. The low-dose test is considered more sensitive in that scenario.4Endocrine Practice. Diagnosing Adrenal Insufficiency: Which Test is Best—The 1-μG or the 250-μG Cosyntropin Stimulation Test?
The classic pass/fail cutoff has long been a peak cortisol of 500 nmol/L (about 18 µg/dL) at 30 minutes. Reach that, and you’re considered to have adequate adrenal function. But this threshold was established with older cortisol assays, and newer immunoassays tend to read lower. One study recalibrating for this found that dropping the cutoff to about 400 nmol/L maintained perfect sensitivity while dramatically improving specificity, meaning fewer false positives and fewer people incorrectly labeled as having adrenal insufficiency.5PubMed Central. Accuracy of the Low-Dose ACTH Stimulation Test for Adrenal Insufficiency Diagnosis: A Re-Assessment of the Cut-Off Value This is important because being told you have adrenal insufficiency when you don’t can mean years of unnecessary steroid replacement.
Distinguishing Primary from Secondary Adrenal Insufficiency
Once you know the adrenals aren’t making enough cortisol, the next question is why. Primary adrenal insufficiency (Addison’s disease) means the adrenal glands themselves are damaged or destroyed. Secondary adrenal insufficiency means the pituitary gland isn’t sending enough ACTH to drive them. The distinction matters because the two conditions are treated differently and have different underlying causes.
A baseline ACTH level, drawn at the same time as the morning cortisol, is the key differentiator.6Clinical Medicine. Diagnosis and management of adrenal insufficiency In primary adrenal insufficiency, the pituitary senses low cortisol and ramps up ACTH production, so ACTH will be elevated, often dramatically. In secondary adrenal insufficiency, the pituitary is the problem, so ACTH will be low or inappropriately normal despite low cortisol. A very low morning cortisol combined with a high ACTH is often enough to diagnose primary adrenal insufficiency without any stimulation test at all.7PubMed Central. Diagnosis and management of adrenal insufficiency
Renin and aldosterone levels add further information. The adrenal cortex also produces aldosterone under the control of the renin-angiotensin system, and in primary adrenal insufficiency that capacity is lost along with cortisol production. A high renin with low aldosterone supports a primary diagnosis. Secondary adrenal insufficiency usually spares aldosterone production because that pathway doesn’t depend on pituitary ACTH.
The Insulin Tolerance Test and Metyrapone Test
The insulin tolerance test is considered the gold standard for diagnosing secondary adrenal insufficiency, particularly when pituitary disease is suspected.7PubMed Central. Diagnosis and management of adrenal insufficiency Insulin is injected intravenously to drive blood sugar low enough to trigger a stress response. If the entire hypothalamic-pituitary-adrenal axis is intact, cortisol should rise sharply. The test is unpleasant and carries real risk for people with seizure disorders or heart disease, so it’s used selectively and always under close medical supervision. It also simultaneously tests growth hormone reserve, making it particularly useful after pituitary surgery or radiation.
The metyrapone test offers another route. Metyrapone blocks the final step in cortisol synthesis, causing cortisol to drop. A healthy pituitary responds by pushing out more ACTH, and the cortisol precursor 11-deoxycortisol accumulates. If 11-deoxycortisol stays low after metyrapone, the axis isn’t responding properly. One important caveat: measuring only ACTH after metyrapone is not reliable enough. A study of over 100 patients found that about 20% of those with confirmed secondary adrenal insufficiency still had ACTH values above the highest threshold proposed in the literature, making ACTH alone misleading. The 11-deoxycortisol measurement is essential.8PubMed Central. Evaluation of plasma ACTH in the metyrapone test is insufficient for the diagnosis of secondary adrenal insufficiency Availability of metyrapone varies by country, which limits its use in some regions.
Salivary Testing as an Emerging Option
Blood draws require a clinic visit, a needle, and careful timing. Salivary cortisol measurement sidesteps some of these hassles because saliva can be collected at home, and what it measures is the “free” (unbound) fraction of cortisol, which is the biologically active portion. This makes it less vulnerable to certain confounders that plague blood-based total cortisol testing.
One study found that an early-morning salivary cortisol value could sort about a third of patients into clear diagnostic categories, either confirming or excluding adrenal insufficiency, without needing any stimulation test at all.9PubMed Central. Salivary cortisol in the diagnosis of adrenal insufficiency: cost efficient and patient friendly Salivary cortisol measured after stimulation testing can also be used as a readout, with post-stimulation values above roughly 8 to 13 nmol/L (depending on time point) showing good diagnostic accuracy.10The Journal of Clinical Endocrinology & Metabolism. Utility of Salivary Cortisol and Cortisone in the Diagnostics of Adrenal Insufficiency
Perhaps more exciting is the measurement of salivary cortisone rather than cortisol. Cortisone is generated from cortisol in the salivary glands and is present in saliva at higher concentrations, making it easier to measure accurately. A study published in NEJM Evidence reported that a single waking salivary cortisone measurement could provide information equivalent to a full ACTH stimulation test in about 70% of participants, with an area under the curve of 0.95, indicating excellent discrimination.11PubMed. Waking Salivary Cortisone to Screen for Adrenal Insufficiency If validated more broadly, this could eventually replace the stimulation test for many patients, though it isn’t yet part of standard clinical guidelines.
When Cortisol Results Mislead
Most cortisol tests in routine clinical practice measure total cortisol, which includes both the free (active) fraction and the portion bound to proteins, primarily corticosteroid-binding globulin (CBG) and albumin. Anything that changes CBG levels will change total cortisol readings without changing how much active cortisol your body actually has. This is a major source of diagnostic error.
Oral contraceptives are the most common culprit. Estrogen-containing contraceptives raise CBG substantially, which inflates total cortisol levels. A woman on the pill might have a normal or even high total cortisol reading despite genuinely insufficient cortisol activity, leading her doctor to wrongly dismiss adrenal insufficiency.12Endocrinology and Metabolism. Clinical and Technical Aspects in Free Cortisol Measurement The reverse problem occurs in critical illness and conditions causing low albumin: CBG drops, total cortisol falls, and clinicians may diagnose adrenal insufficiency that isn’t really there. These CBG-related distortions also affect the pharmacokinetics of hydrocortisone replacement therapy, since the bound fraction acts as a buffer regulating how long cortisol stays in circulation.13PubMed. Corticosteroid-binding globulin regulates cortisol pharmacokinetics
Assay interference is another issue. Direct immunoassays for cortisol can cross-react with cortisol metabolites and synthetic steroids like prednisolone or dexamethasone.14Annals of Laboratory Medicine. Hormone Immunoassay Interference: A 2021 Update If you’re being tested while tapering off prednisone, for instance, the assay might pick up the synthetic steroid and return a falsely reassuring cortisol value. Your doctor should be aware of which steroids the lab’s assay can distinguish and time testing accordingly.
Testing during critical illness deserves special mention. What constitutes “enough” cortisol during a severe infection or after major surgery is genuinely unclear, and the standard stimulation test loses much of its reliability in this setting. Changes in cortisol distribution, CBG suppression, and altered tissue sensitivity all conspire to make results difficult to interpret.15Acute and Critical Care. Critical illness-related corticosteroid insufficiency: latest pathophysiology and management guidelines The concept of “critical illness-related corticosteroid insufficiency” uses different criteria than outpatient adrenal testing, and the definition itself has shifted over the years.16PubMed Central. The diagnosis of adrenal insufficiency in the critically ill patient: does it really matter?
How Shift Work Complicates Morning Cortisol
The instruction to “get a morning cortisol” assumes you sleep at night and wake in the morning. For shift workers, that assumption doesn’t hold, and the cortisol rhythm gets complicated. The underlying circadian clock still tries to peak cortisol in the early morning hours regardless of when you sleep, but waking cortisol after daytime sleep following a night shift is generally lower than waking cortisol after normal nighttime sleep.17PubMed Central. Effect of night-shift work on cortisol circadian rhythm and melatonin levels One study of female hospital employees found that the cortisol awakening response was significantly blunted after day sleep compared to night sleep, and that this effect was most pronounced in early chronotypes, people who are naturally “morning people.”18Scientific Reports. Night work, chronotype and cortisol at awakening in female hospital employees
A longitudinal study following junior physicians over a year found that those doing shift work had higher waking cortisol and steeper daily cortisol slopes at follow-up compared to those on day shifts, suggesting longer-term alterations in the rhythm.19PubMed Central. Impact of shift work on the diurnal cortisol rhythm: a one-year longitudinal study in junior physicians The key practical point: if you work night shifts, mention this when your doctor orders cortisol testing. A “morning” cortisol drawn at 8 a.m. when you normally wake at 3 p.m. is not capturing your cortisol peak and could produce a falsely low result that triggers unnecessary further workup.
Testing in Children
Children aren’t small adults when it comes to cortisol testing. The stimulation test is used, but the interpretation thresholds differ and the gray zone is wider. A pediatric-focused analysis found that a 30-minute cortisol below about 440 nmol/L (16 µg/dL) was highly predictive of central adrenal insufficiency, while values above roughly 600 nmol/L (22 µg/dL) virtually excluded it. Everything in between was diagnostically uncertain.20PubMed Central. Pitfalls in the Diagnosis of Central Adrenal Insufficiency in Children That indeterminate range is wide enough to capture a significant fraction of children being tested, which means pediatric endocrinologists often need additional information, clinical context, repeat testing, or alternative tests to make the call.
Children receiving inhaled corticosteroids for asthma are a particularly common group where the question of adrenal suppression arises. Even “local” inhaled steroids can suppress the adrenal axis, so clinicians should have a lower threshold for testing in children on moderate to high doses of inhaled steroids who develop unusual fatigue, growth deceleration, or hypoglycemia.
Steroid-Induced Adrenal Insufficiency and Recovery Testing
By far the most common cause of secondary adrenal insufficiency in clinical practice is prior use of exogenous glucocorticoids. As many as 1 to 3% of the general population are on glucocorticoid therapy at any given time, and prolonged use suppresses the body’s own cortisol production. The risk isn’t limited to oral steroids: inhaled, topical, nasal, intra-articular, and even short courses (under four weeks) or low doses (below 5 mg prednisone equivalent daily) can suppress the axis.21PubMed Central. Glucocorticoid Withdrawal-An Overview on When and How to Diagnose Adrenal Insufficiency in Clinical Practice
When glucocorticoids are tapered and eventually stopped, the question becomes whether the adrenal axis has recovered enough to sustain you through normal life and illness. This is assessed with interval stimulation tests, usually repeated every 6 to 12 months until the result normalizes. Recovery is defined by a stimulated cortisol at or above about 430 nmol/L (roughly 15.6 µg/dL). Patients whose axis hasn’t recovered are maintained on the lowest possible physiological replacement dose, and if they’re symptomatic during the taper, they may be switched from longer-acting prednisolone to hydrocortisone to better mimic the body’s natural rhythm.22The Journal of Clinical Endocrinology & Metabolism. A Retrospective Study on Weaning Glucocorticoids and Recovery of the Hypothalamic–Pituitary–Adrenal Axis Recovery can take months or even years in some patients, and there’s no reliable way to predict at the outset who will bounce back quickly.
Finding the Cause After Diagnosis
Confirming adrenal insufficiency is only half the job. Identifying what’s behind it guides long-term management and surveillance for related conditions. In primary adrenal insufficiency, the dominant cause in high-income countries is autoimmune destruction of the adrenal cortex, and the marker for this is the 21-hydroxylase autoantibody. Over 90% of patients with autoimmune Addison’s disease test positive for this antibody, and remarkably, most remain positive for decades after diagnosis.23PubMed Central. The natural history of 21-hydroxylase autoantibodies in autoimmune Addison’s disease Antibody levels do decline slowly over time, with positivity eventually dropping to around 75% in patients who have had the disease for more than 30 years.24European Journal of Endocrinology. The natural history of 21-hydroxylase autoantibodies in autoimmune Addison’s disease
Interestingly, the same antibody has predictive value even before clinical Addison’s disease develops. In people at high risk, such as those with other autoimmune conditions like type 1 diabetes or autoimmune thyroid disease, 21-hydroxylase autoantibody levels are significantly higher in those who eventually progress to adrenal failure compared to those who don’t.25PubMed Central. Predicting the onset of Addison’s disease: ACTH, renin, cortisol and 21-hydroxylase autoantibodies If the antibody is negative, other causes need to be considered, including infections like tuberculosis (still a leading cause in many parts of the world), adrenal hemorrhage, infiltrative diseases, and genetic conditions like adrenoleukodystrophy, particularly in younger men.
For secondary adrenal insufficiency not caused by steroid withdrawal, imaging of the pituitary and hypothalamus with MRI is standard. Pituitary tumors, surgical damage, radiation therapy, and inflammatory conditions like lymphocytic hypophysitis can all impair ACTH secretion. Many of these conditions affect other pituitary hormones as well, so a complete pituitary hormone panel is typically ordered alongside the cortisol workup.
Hair Cortisol for Monitoring Replacement Therapy
Once someone is on glucocorticoid replacement, the challenge shifts from diagnosis to monitoring. Blood and saliva give you a snapshot of cortisol at one point in time, but what you really want to know is whether replacement dosing is adequate over weeks and months. Hair cortisol measurement offers a window into longer-term cortisol exposure because cortisol is incorporated into hair as it grows, and a centimeter of hair roughly reflects about a month of cumulative exposure.
A study comparing patients with adrenal insufficiency on hydrocortisone replacement to healthy controls found that hair cortisol correlated with the glucocorticoid dose, and that male patients actually had higher hair cortisol than male controls, suggesting possible overreplacement.26PubMed. Hair cortisol content in patients with adrenal insufficiency on hydrocortisone replacement therapy This isn’t yet part of routine clinical practice, but it represents a promising direction for detecting chronic over- or under-replacement that single blood draws would miss. Overreplacement matters because it carries the same long-term risks as taking too much corticosteroid deliberately: bone loss, metabolic changes, and cardiovascular effects.