Free T3 Test: What It Shows and What It Does Not
Quick answer: Free T3 measures the more active thyroid hormone that is not attached to proteins. It is often added when an overactive thyroid is suspected or when TSH and free T4 do not fully explain symptoms.
About this test
T3 (triiodothyronine) is made mostly from T4 in the body. Free T3 is the part that is not attached to proteins and can act on tissues.
Why is it ordered?
- Looking at symptoms or results that suggest an overactive thyroid.
- Following up an abnormal TSH or free T4.
- Reviewing thyroid function alongside antibody tests.
What results may show
- A high free T3 can go along with an overactive thyroid.
- A low free T3 can occur with an underactive thyroid or with serious illness.
What this test may not show
- It does not show the cause of a thyroid problem.
- It is less useful alone than together with TSH and free T4.
- Serious illness, some medicines and biotin can change results.
- An abnormal result does not always mean treatment is needed.
How to prepare
- No special preparation is usually needed.
- Stop biotin and hair, skin and nail supplements at least 72 hours before your draw. Do not stop a prescribed medicine without asking.
Where you can get this test
| Option | Price | Includes |
|---|---|---|
| Comprehensive Thyroid Panel | $119 | TSH, free T4, free T3, TPO antibodies, thyroglobulin antibodies |
A doctor’s visit is not required to order. Order online, then visit our Buffalo Grove or Lake Zurich location for the sample. Review results with your own healthcare provider.
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Frequently asked questions
What is the difference between T3 and T4?
T4 is the main hormone the thyroid makes. T3 is the more active form, made mostly from T4.
Is free T3 part of routine thyroid testing?
Usually TSH and free T4 come first. Free T3 is added when more detail is needed.
Does biotin affect free T3?
It can change thyroid test results. Stop it at least 72 hours before your draw.
Sources
Medically reviewed by Rodion Levin, FNP-C, Founder and Medical Director. This page is for general education. It does not diagnose a condition or replace advice from your healthcare provider.
