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Hormone testing guide · 2026
Cortisol is the most rhythm-dependent hormone in common use. Test it at the wrong hour and the number is not wrong so much as meaningless.
A serum cortisol test should be drawn between 8am and 9am, when the daily peak occurs. Cortisol follows a strong circadian rhythm, falling through the day to a trough around midnight, so a mid-afternoon result cannot be compared against a morning reference range. Screening for cortisol excess (Cushing’s syndrome) uses late-night salivary cortisol, 24-hour urinary free cortisol or an overnight dexamethasone suppression test rather than a single morning draw. Cortisol testing cannot diagnose “adrenal fatigue”, which is not a recognised medical condition.
| Serum timing | 8–9am, seated and rested, after a normal night’s sleep |
|---|---|
| Screening for excess | Late-night salivary cortisol, 24-hour urinary free cortisol, or overnight dexamethasone suppression |
| Screening for deficiency | Early-morning serum cortisol, with dynamic testing if borderline |
| Interferes with results | Oestrogen and combined oral contraceptives raise cortisol binding globulin; shift work shifts the rhythm |
| Collection | At home in Bangkok or at 20+ MedEx sites nationwide; nurse-at-home draw available |
| Interpretation | Doctor consultation or specialist teleconsultation to review the report |
Cortisol peaks shortly after waking and declines across the day. The difference between the morning peak and the late-evening trough is severalfold, which is far larger than most of the differences clinicians are looking for.
Two practical consequences follow. First, a serum cortisol is only interpretable with a documented draw time, ideally 8–9am. Second, night-shift workers and people with substantially displaced sleep have a shifted rhythm, and the standard timing does not apply to them — the clinician needs to know your actual sleep schedule.
Acute illness, pain, recent surgery and significant psychological stress all raise cortisol appropriately. Testing during an acute episode measures the stress, not the axis.
Adrenal insufficiency presents with fatigue, weight loss, nausea, dizziness on standing, salt craving and, in primary disease, skin darkening. It can be life-threatening in a crisis, so genuine suspicion warrants prompt medical assessment rather than a self-ordered panel.
The first-line test is an early-morning serum cortisol. A clearly normal result makes adrenal insufficiency unlikely; a clearly low one supports it. Values in between require a dynamic test, typically a short Synacthen (ACTH stimulation) test, arranged through a specialist.
ACTH is measured alongside to separate primary adrenal disease from pituitary causes. If you are taking any glucocorticoid — including inhaled, topical, or joint injections — tell the clinician, as these suppress the axis and change interpretation. A specialist teleconsultation is the appropriate route for this pathway.
Book an 8-9am cortisol draw at home or at a MedEx site.
Cushing’s syndrome is suspected with features such as rapid central weight gain, purple striae, easy bruising, proximal muscle weakness, new or difficult-to-control hypertension and diabetes, and skin thinning.
A single morning serum cortisol is a poor screening test for it, because morning values overlap heavily between affected and unaffected people. The recognised screening options are:
Any positive screen needs specialist confirmation. Note that oestrogen therapy and combined oral contraceptives raise cortisol binding globulin and therefore total serum cortisol, without excess free hormone.
Adrenal fatigue proposes that chronic stress exhausts the adrenal glands, producing tiredness that a four-point salivary cortisol curve can detect. Systematic reviews of the published studies have found no consistent relationship between cortisol profiles and fatigue, and the concept is not recognised by endocrine societies.
That does not mean the fatigue is imaginary. It means a cortisol curve is unlikely to explain it. Genuinely common and treatable causes include iron deficiency, thyroid disease, obstructive sleep apnoea, poor sleep quality, depression and anxiety, uncontrolled blood sugar, vitamin D deficiency and medication effects.
A more productive first step is a broad health checkup package covering full blood count, ferritin, TSH and free T4, HbA1c, vitamin D, liver and kidney function — with cortisol added only if there is a clinical reason.
Between 8am and 9am, seated and rested, after a normal night’s sleep. Cortisol peaks in the early morning and falls through the day, so an afternoon sample cannot be compared against a morning reference range.
No. Adrenal fatigue is not a recognised medical diagnosis and systematic reviews have found no consistent link between cortisol profiles and fatigue. Persistent tiredness is better investigated with a broader panel covering iron, thyroid, blood sugar, vitamin D and sleep.
Yes, for one specific purpose: late-night salivary cortisol is a recognised screening test for cortisol excess, because it detects loss of the normal night-time trough. Four-point daytime saliva curves for fatigue are not validated.
Yes. Oestrogen and combined oral contraceptives raise cortisol binding globulin, which raises total serum cortisol without a true excess of free hormone. Tell the clinician what you are taking.
Yes. Glucocorticoids in any form, including inhaled, topical and injected preparations, suppress the axis and alter interpretation. Never stop a prescribed steroid without medical advice.
Fasting is not required for cortisol itself, but morning appointments are often combined with fasting tests such as glucose and lipids.
Persistent fatigue is usually explained by a broader panel than cortisol alone.
Medical disclaimer: This article is general health information and does not replace medical advice, diagnosis or treatment. Laboratory reference ranges differ between laboratories and results must be interpreted alongside your symptoms, medicines and medical history. Speak with a qualified clinician before starting, stopping or changing any treatment. Service details, inclusions and prices can change — confirm them with MedEx before booking.
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