About

Cortisol is often called the "stress hormone," but it plays a vital role in regulating metabolism, reducing inflammation, and controlling the sleep-wake cycle. A key characteristic of cortisol is its diurnal rhythm: levels should naturally be highest in the morning (to help you wake up) and gradually drop throughout the day, reaching their lowest point around midnight (to allow for sleep).

Single-point measurements can be misleading if not interpreted in the context of time. A "high" level at 8 AM might be completely normal, whereas that same level at 10 PM could indicate Cushing's syndrome or chronic stress. This tool helps patients and practitioners contextualize lab results by comparing input values against time-specific reference ranges and converting between international measurement units.

Formulas

Cortisol units are converted using the molecular weight of cortisol (362.46 g/mol):

1 µg/dL = 27.59 nmol/L

Interpretation Logic:

Step 1. Standardize input to nmol/L.
Step 2. Select reference range based on time (Morning, Afternoon, Evening).
Step 3. Compare: If < Min (Low/Adrenal Insufficiency), If > Max (High/Hypercortisolism), Else (Normal).

Reference Data

Time of Collection	Range (µg/dL)	Range (nmol/L)	Clinical Context
Morning (8 AM)	6.0 - 23.0	166 - 635	Peak adrenal output.
Afternoon (4 PM)	3.0 - 16.0	83 - 441	Declining phase.
Evening (Midnight)	< 5.0	< 138	Nadir (Lowest point).
Stress/Illness	Elevated	Elevated	Acute response masking rhythm.

Frequently Asked Questions

High evening cortisol can be caused by chronic stress, Cushing's syndrome, sleep apnea, or simply drinking caffeine or exercising too late in the day. It is a common cause of insomnia.

Total cortisol measures both bound (attached to proteins) and free cortisol in the blood. Free cortisol is the active form. Most standard blood tests measure total cortisol, while saliva tests measure free cortisol.

8 AM is the standard because that is when cortisol secretion typically peaks. It provides a baseline to assess the adrenal gland's maximum output capacity.

Divide the nmol/L value by 27.59. For example, 500 nmol/L / 27.59 ≈ 18.1 µg/dL.