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Serum/plasma free cortisol concentration (mass concentration)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of free (unbound) cortisol in your blood, which helps assess cortisol levels independent of proteins that normally carry cortisol in the blood.

Quick summary

["Free cortisol helps diagnose conditions like Cushing’s syndrome or adrenal insufficiency by evaluating active cortisol levels."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
47770-3
Short code
GH 30M p Ins IV SerPl-aCnc
Synonyms
GH 30M p Ins IV SerPl-aCnc
Category
hormone
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your healthcare provider may order this test if you have symptoms of hormonal imbalances, such as unexplained weight changes, fatigue, or unusual mood swings. It can also be used to monitor conditions like Cushing’s syndrome or adrenal disorders.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the test if serum/plasma is collected. If urine is involved (e.g., 24-hour collection), follow your provider’s instructions carefully.

How the Test Is Performed

A blood sample is drawn from a vein in your arm, or urine may be collected over 24 hours for certain cortisol measurements. The sample is then analyzed in a lab using techniques like immunoassay or chromatography to measure cortisol levels.

Possible Results and Interpretation

Results are compared to reference ranges, but individual ranges can vary by lab. High free cortisol may indicate Cushing’s syndrome, while low levels could suggest adrenal insufficiency or other hormonal issues.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Free cortisol testing is critical for diagnosing conditions affecting cortisol production or metabolism. Elevated levels may signal overproduction (e.g., from tumors or excess steroid use), while low levels can indicate adrenal gland dysfunction or pituitary disorders. This test helps guide treatment plans for conditions like Addison’s disease or secondary Cushing’s syndrome.

Short name

Free Cortisol

Frequently asked questions

What does a high free cortisol level mean?
A high free cortisol level may suggest Cushing’s syndrome, which can be caused by tumors (such as pituitary or adrenal), long-term steroid use, or other hormonal imbalances. Your doctor will review symptoms and possibly additional tests to determine the cause.
What does a low free cortisol level mean?
Low free cortisol levels often point to adrenal insufficiency (e.g., Addison’s disease) or problems with the pituitary gland regulating cortisol. This may require further evaluation, including hormone replacement therapy or investigation of underlying causes like autoimmune disorders or infections.
Can diet or medications affect free cortisol test results?
Yes, certain medications (e.g., steroids, antidepressants) or dietary factors (e.g., caffeine, alcohol) can influence cortisol levels. If you’re taking medications or have recent changes in diet, your provider may ask you to avoid them before testing or adjust interpretation accordingly.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy