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Free triiodothyronine (T3) concentration in serum/plasma

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

This test measures the amount of active thyroid hormone (T3) in your blood that is not bound to proteins, helping assess your thyroid function.

Quick summary

["Your thyroid hormone levels affect metabolism, energy, and overall health—this test helps check if they’re working properly."]

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

Key facts at a glance

LOINC
35230-2
Short code
T3Free SerPl-msCnc
Synonyms
T3Free SerPl-msCnc
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
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 It’s Done

Your doctor may order a free T3 test if you have symptoms like fatigue, weight changes, or unexplained heart issues, or if other thyroid tests (like TSH) suggest an issue. It can also be used to monitor thyroid conditions like hypothyroidism or hyperthyroidism.

What to Expect

A small blood sample is taken from a vein in your arm. You may feel a slight pinch, but it usually doesn’t cause discomfort. The test is quick and done in a lab.

How It Works

Your blood sample is analyzed to separate the free (unbound) T3 from protein-bound T3. The free portion is measured to reflect how much of this hormone is available to affect your body’s systems.

What Results Mean

Results help doctors diagnose or manage thyroid disorders. While reference ranges vary by lab, very low or very high levels may indicate conditions like underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid function.

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.

Free T3 is important because it directly influences metabolism, heart rate, and brain function. Abnormal levels can signal thyroid dysfunction, which may require treatment like medication or lifestyle adjustments.

Short name

Free T3

Frequently asked questions

What causes a free T3 test?
Your doctor might recommend it if you have symptoms like unexplained weight changes, fatigue, or heart palpitations, or if your thyroid-stimulating hormone (TSH) test results are abnormal.
How long does it take to get results?
Results are typically available within 1 to 3 business days, depending on your lab and insurance processing times.
Can I eat or drink before this test?
You can eat and drink normally before the test, as it doesn’t require fasting. However, always follow your doctor’s specific instructions for your appointment.
  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