Cortisol Conversion Calculator
Convert cortisol units between mcg/dL, nmol/L, and ng/mL for lab interpretation. Enter values for instant results with step-by-step formulas.
Formula
nmol/L = mcg/dL x 27.59 | ng/mL = mcg/dL x 10
Cortisol has a molecular weight of 362.46 g/mol. The conversion factor of 27.59 is derived from: (1 mcg/dL) x (10 mcg/L per mcg/dL) x (1 mol / 362,460 mcg) x (1,000,000,000 nmol / 1 mol) x (1 L) = 27.59 nmol/L. The mcg/dL to ng/mL conversion is a simple factor of 10.
Worked Examples
Example 1: Converting Morning Cortisol for International Reference
Problem: A patient has a morning serum cortisol of 18 mcg/dL from a US lab. An international specialist requests the value in nmol/L to compare with their reference ranges.
Solution: Cortisol in nmol/L = value in mcg/dL x 27.59\n18 mcg/dL x 27.59 = 496.62 nmol/L\nAlso: 18 mcg/dL x 10 = 180 ng/mL\nMorning reference range: 171 - 535 nmol/L\nResult falls within normal morning range.
Result: 18 mcg/dL = 496.62 nmol/L = 180 ng/mL (Normal morning range)
Example 2: Evaluating a Low Cortisol from European Lab
Problem: A patient presents with fatigue and has a morning cortisol of 110 nmol/L from a European laboratory. Convert to mcg/dL to compare with US reference standards.
Solution: Cortisol in mcg/dL = value in nmol/L / 27.59\n110 / 27.59 = 3.99 mcg/dL\nAlso: 3.99 mcg/dL x 10 = 39.9 ng/mL\nUS morning reference: 6.2 - 19.4 mcg/dL\nResult is BELOW the normal morning range, suggesting possible adrenal insufficiency.
Result: 110 nmol/L = 3.99 mcg/dL = 39.9 ng/mL (Below normal morning range)
Frequently Asked Questions
What are the different units used to measure cortisol levels?
Cortisol is measured in three primary units across different laboratory systems worldwide. The most common unit in the United States is micrograms per deciliter (mcg/dL), which is a conventional unit used in most American hospital and commercial laboratories. The SI (International System) unit is nanomoles per liter (nmol/L), which is standard in Europe, Canada, Australia, and many other countries. Nanograms per milliliter (ng/mL) is a third unit sometimes encountered in research settings and certain specialized assays. Understanding these different units is essential for correctly interpreting cortisol results from international publications and different lab systems.
How do you convert cortisol between mcg/dL and nmol/L?
The conversion between mcg/dL and nmol/L is based on the molecular weight of cortisol (hydrocortisone), which is 362.46 grams per mole. To convert from mcg/dL to nmol/L, you multiply by 27.59. To convert from nmol/L to mcg/dL, you divide by 27.59. For example, a cortisol level of 15 mcg/dL equals approximately 413.9 nmol/L. This conversion factor is universally accepted and is derived from the formula: nmol/L = mcg/dL multiplied by 10 (to convert to mcg/L) divided by molecular weight (362.46) multiplied by 1,000,000 (to convert to nmol). Memorizing the factor 27.59 makes quick mental conversions possible in clinical practice.
What is the normal morning cortisol reference range?
Normal morning cortisol levels, typically measured between 6 AM and 8 AM when cortisol peaks due to the circadian rhythm, range from approximately 6.2 to 19.4 mcg/dL (171 to 535 nmol/L) in most laboratory reference ranges. These values can vary slightly between laboratories depending on the assay method used, such as immunoassay versus liquid chromatography-tandem mass spectrometry (LC-MS/MS). Morning cortisol below 3 mcg/dL (83 nmol/L) strongly suggests adrenal insufficiency, while values above 18 mcg/dL (497 nmol/L) generally rule it out. The timing of blood collection is critical because cortisol follows a strong diurnal pattern, peaking 30 to 60 minutes after waking and declining throughout the day.
Why does cortisol follow a diurnal pattern throughout the day?
Cortisol secretion follows a circadian rhythm controlled by the hypothalamic-pituitary-adrenal (HPA) axis and the suprachiasmatic nucleus, which is the brain's master circadian clock. Cortisol levels are highest in the early morning, typically peaking between 6 AM and 8 AM, to prepare the body for waking activities by mobilizing glucose and modulating immune function. The cortisol awakening response (CAR) causes an additional 50 to 75 percent surge within 30 to 45 minutes of waking. Levels then decline progressively throughout the day, reaching their nadir around midnight. This diurnal variation means that a single cortisol measurement must always be interpreted in the context of the time it was drawn.
What conditions cause elevated cortisol levels?
Elevated cortisol, known as hypercortisolism or Cushing syndrome, can result from several causes. Endogenous causes include pituitary adenomas secreting excess ACTH (Cushing disease, accounting for about 70 percent of endogenous cases), adrenal tumors producing cortisol autonomously, and ectopic ACTH-secreting tumors such as small cell lung carcinoma. Exogenous causes, which are the most common overall, include chronic use of glucocorticoid medications like prednisone and dexamethasone. Pseudo-Cushing states from severe depression, alcoholism, or morbid obesity can also cause mildly elevated cortisol. Screening tests include 24-hour urinary free cortisol, late-night salivary cortisol, and the 1 mg overnight dexamethasone suppression test.
What conditions cause low cortisol levels?
Low cortisol levels indicate adrenal insufficiency, which is classified as primary, secondary, or tertiary. Primary adrenal insufficiency (Addison disease) results from destruction of the adrenal cortex itself, most commonly from autoimmune adrenalitis in developed countries or tuberculosis in developing nations. Secondary adrenal insufficiency occurs from insufficient ACTH secretion by the pituitary gland, often due to pituitary tumors or surgery. Tertiary adrenal insufficiency, the most common form, results from chronic exogenous glucocorticoid use suppressing the HPA axis, which can persist for months after steroid discontinuation. Morning cortisol below 3 mcg/dL is highly suspicious, while values above 18 mcg/dL effectively rule out the diagnosis.