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Free triiodothyronine (T3) concentration in serum/plasma

Published: August 9, 2026Updated: August 17, 2026

This test measures the amount of free (unbound) thyroid hormone T3 in your blood. Free T3 is active and helps regulate metabolism, energy levels, and other bodily functions.

Quick summary

["Free T3 helps assess your thyroid function and whether your body is producing enough active thyroid hormone."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
29239-1
Short code
T3Free SerPl Dialy-mCnc
Synonyms
T3Free SerPl Dialy-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
80
Ref high
200
Critical range
20 N/A – 400 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
dialysis
CPT
84436
SNOMED
302346007
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 Matters

Your thyroid produces hormones that control many important functions in your body, including metabolism, heart rate, and digestion. Free T3 is one of the key hormones your body uses to stay balanced. This test helps doctors check if your thyroid is working properly, especially if you have symptoms like fatigue, weight changes, or unexplained mood swings.

What to Expect Before the Test

No special preparation is needed for this test. You may have a small needle inserted into a vein in your arm, or a blood sample may be drawn from your finger. If you’re taking thyroid medication, let your healthcare provider know so they can adjust timing if needed.

Common Reasons for Testing

Doctors may order a free T3 test if you have symptoms of thyroid problems, such as anxiety, weight loss/gain, or irregular heartbeats. It’s also used to monitor thyroid hormone levels in people with thyroid disease, like hypothyroidism or hyperthyroidism, or if you’re taking thyroid replacement medication.

What the Results Mean

Results alone don’t provide a full picture—your doctor will consider them alongside other tests and your symptoms. Very high or very low levels may signal thyroid issues, but they can also be influenced by other health conditions or medications.

Reference range

Adult: 80-200 ng/dLN/A

Specimen: serum

Take your thyroid medication after the blood draw if applicable. No fasting required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Free T3

Frequently asked questions

How often will I need this test?
This test is often done once or twice a year if you have a thyroid condition, but your doctor will decide how often based on your specific needs. Frequent testing may be needed if you’re adjusting medications or experiencing symptoms that change over time.
What if my results are outside the normal range?
If your free T3 level is outside the normal range, your doctor will investigate further. This could include additional thyroid tests (like T4 or thyroid-stimulating hormone), imaging, or other evaluations to find the cause. They may also check for conditions like thyroiditis or pituitary disorders that can affect hormone levels.
Can diet or supplements affect my free T3 test results?
Yes, certain foods, supplements, or medications (like some antidepressants or blood pressure drugs) can influence thyroid hormone levels. Avoid taking iodine-rich foods (like seafood or supplements) or certain medications 2–3 days before your test, as they may interfere with accuracy.
  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 17, 2026. · Read our editorial policy