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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 an active form of thyroid hormone that helps regulate metabolism.

Quick summary

["Free T3 helps assess how well your thyroid is functioning and whether your thyroid hormone levels are balanced."]

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

Key facts at a glance

LOINC
83126-3
Short code
T3Free SerPl IA-sCnc
Synonyms
T3Free SerPl IA-sCnc
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
ia
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 Might Be Ordered

Your healthcare provider may recommend this test if you have symptoms of thyroid dysfunction, such as fatigue, weight changes, or irregular heartbeats. It could also be used to monitor thyroid hormone levels during treatment for thyroid disorders.

What to Expect Before the Test

No special preparation is usually needed. You may be asked to fast for 8–12 hours before the test if it’s part of a broader thyroid panel. Bring any medications you’re taking to your appointment.

How the Test Works

A blood sample is drawn from a vein in your arm. The sample is then analyzed to measure the concentration of free T3 in your serum or plasma.

What the Results Mean

Results help determine whether your thyroid is producing too much or too little hormone. Abnormal levels may indicate conditions like hypothyroidism or hyperthyroidism.

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.

Clinical Significance: Free T3 is a key indicator of thyroid function. While total T3 includes bound hormone, free T3 reflects the biologically active portion. Low free T3 may signal hypothyroidism, while elevated levels could suggest hyperthyroidism or other metabolic issues.

Short name

Free T3

Frequently asked questions

What causes free T3 levels to be checked?
Your doctor may order this test if you have symptoms like unexplained weight changes, fatigue, or heart palpitations, or if you’re being monitored for thyroid disorders.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on your lab’s processing time.
Can I eat or drink before this test?
If fasting is required (common for thyroid panels), avoid food and liquids (except water) for 8–12 hours before the test. Always follow your provider’s instructions.
  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