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Free triiodothyronine (T3) in serum/plasma

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

This test measures the amount of free thyroid hormone T3 in your blood. Free T3 is the active form of thyroid hormone that your body uses directly.

Quick summary

["Your thyroid hormone levels help regulate metabolism, energy, and overall health."]

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

Key facts at a glance

LOINC
25805-3
Short code
T3Free sp3 p chal SerPl-sCnc
Synonyms
T3Free sp3 p chal SerPl-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
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 like fatigue, weight changes, or irregular heart rhythms. It could also be part of a thyroid function evaluation, especially if you have conditions like hypothyroidism or hyperthyroidism, or if you’re taking thyroid medication.

What to Expect Before the Test

No special preparation is needed for this blood test. You may have a small needle inserted into your arm or hand to draw blood. If you’re on thyroid medication, let your provider know before the test.

What the Results Mean

Free T3 levels help assess whether your thyroid is producing enough active hormone. Abnormal results may indicate thyroid dysfunction, which can affect metabolism and energy levels.

Common Conditions Linked to Free T3

Changes in free T3 levels can be linked to thyroid disorders, such as hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid). It may also be checked in patients with pituitary disorders or those on thyroid-replacement therapy.

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 Level

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, anxiety, or heart palpitations. It’s also part of routine thyroid function testing, especially if you have a family history of thyroid disorders or are taking thyroid medication.
How long does it take to get the results?
Results are typically available within 1 to 3 business days, depending on your lab’s processing time. Your healthcare provider will discuss the findings with you during your follow-up visit.
Can I eat or drink anything before this test?
No dietary restrictions apply. You can eat and drink normally before the blood draw, unless your provider has specific 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