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Endocrine
Serum cortisol (morning)
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The main glucocorticoid of the adrenal cortex, secreted in a circadian rhythm with a peak shortly after waking and a nadir around midnight. A single morning value is a screening result and is interpreted alongside dynamic testing.
Reference interval
| Conventional (µg/dL) | SI (nmol/L) | |
|---|---|---|
| Adults | 6–23 µg/dL | 165–635 nmol/L |
Specimen: Serum, sampled at about 08:00 to 09:00 · Conventional × 27.59 = SI
Causes of high values
Cushing syndrome (pituitary, ectopic ACTH or adrenal), stress and acute illness, depression, obesity, oestrogen therapy or pregnancy (raised binding globulin), chronic alcohol use. Loss of the diurnal rhythm is typical of Cushing syndrome.
Causes of low values
Adrenal insufficiency (Addison disease, autoimmune adrenalitis, tuberculosis), secondary adrenal insufficiency (pituitary disease, prolonged glucocorticoid therapy), hypopituitarism. A very low morning value supports adrenal insufficiency and a very high one largely excludes it.
Pre-analytical and method notes
The 6 to 23 µg/dL (about 165 to 635 nmol/L) range is wide and assay-dependent, and the result must be interpreted with dynamic tests (ACTH stimulation, dexamethasone suppression, late-night salivary cortisol), whose cut-offs follow guidelines. Oestrogens raise total cortisol through cortisol-binding globulin, and hydrocortisone and prednisolone cross-react in some assays. Reference intervals vary by laboratory, method and population; use the interval printed on the report.
References
- Williams Textbook of Endocrinology. Elsevier, 14th edition (2019) — Tier 3 (textbook)
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
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