Free T4 Test: What It Shows and What It Does Not
Quick answer: Free T4 measures the active form of the main hormone your thyroid makes. It is usually ordered with TSH. Together they show both the signal to the thyroid and the hormone it is putting out.
About this test
T4 (thyroxine) is the main hormone the thyroid releases. Free T4 is the part not attached to proteins, which is the part that reaches your tissues. It is considered more accurate than total T4 and is used more often.
Why is it ordered?
- Following up an abnormal TSH.
- Looking at symptoms of an underactive or overactive thyroid.
- Checking whether thyroid hormone medicine is working.
- Looking at thyroid nodules or pituitary conditions.
What results may show
- A high free T4 can go along with an overactive thyroid, too much thyroid medicine or excess iodine.
- A low free T4 can go along with an underactive thyroid, some stages of thyroiditis or a pituitary problem.
What this test may not show
- It cannot diagnose thyroid disease by itself. It is read with TSH and sometimes free T3 and antibodies.
- Severe illness, pregnancy, birth control pills and steroids can change T4 levels.
- Biotin can change results. Stop it at least 72 hours before your draw.
- An abnormal result does not always mean treatment is needed.
How to prepare
- No special preparation is usually needed.
- Tell your provider about medicines and supplements. Stop biotin at least 72 hours before your draw.
- Do not stop a medicine without asking your prescriber.
Where you can get this test
| Option | Price | Includes |
|---|---|---|
| Thyroid Function Test (TSH and Free T4) | $60 | TSH and free T4 |
| Comprehensive Thyroid Panel | $119 | TSH, free T4, free T3, TPO antibodies, thyroglobulin antibodies |
| Hair Loss Panel | $199 | CBC, ferritin, iron and TIBC, TSH, free T4, vitamin D, total testosterone, DHEA-S |
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.
Related tests
- TSH: TSH is the hormone your pituitary gland makes to tell your thyroid how much hormone to produce.
- DHEA-S: DHEA-S is a hormone made mostly by the adrenal glands that the body uses to make testosterone and estrogen.
- Browse the full Lab Test Library
Frequently asked questions
What is the difference between free T4 and total T4?
Free T4 is the active part not attached to proteins. Total T4 includes both free and attached hormone.
Why is free T4 ordered with TSH?
TSH shows the signal to the thyroid. Free T4 shows the hormone the thyroid is producing.
Can medicines change free T4?
Yes. Steroids, birth control pills and others can. Tell your provider what you take.
Do I need to fast?
Usually not. Follow any instruction given when you order.
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.
