| Marker | Gabriel optimal | Typical lab range | Functional range |
|---|---|---|---|
| Free T4 | 1 to 1.5 ng/dL | 0.8 to 1.8 | 1 to 1.5 |
What free T4 measures
Your thyroid makes thyroxine (T4). In the blood, some T4 is attached to proteins, which keeps it from entering tissues. The unattached part, free T4, is the active form that enters body tissues where it is needed. See the free T4 lab guide for more.
Clinicians order it to check for an underactive or overactive thyroid, to look into a goiter or thyroid nodules, to check pituitary problems, and to see whether thyroid hormone medicine is at the right dose. Newborns are also screened with T4 for congenital hypothyroidism.
Free T4 vs total T4
- Free T4 measures only the unbound hormone. Experts consider it more accurate, so it is the one ordered most often.
- Total T4 measures bound and free T4 together. Pregnancy, birth control pills, steroids, severe illness and inherited differences in the binding proteins can all change T4 results.
- Some labs report a free T4 index, an estimate calculated from total T4.
Reading free T4 with TSH
| TSH | Free T4 | What it can mean |
|---|---|---|
| High | Low | An underactive thyroid (hypothyroidism), often Hashimoto disease |
| Low | High | An overactive thyroid (hyperthyroidism), such as Graves disease, thyroiditis, toxic nodules, or too much thyroid medicine |
| Low or normal | Low | Less common: a pituitary problem, or an effect of severe illness |
| Normal | Normal | Thyroid hormone levels are in range. A normal free T4 does not always rule out a thyroid problem, so a clinician may add other tests |
What can cause a high or low free T4
Low free T4 can come from Hashimoto disease and other causes of an underactive thyroid, some stages of thyroiditis, too much or too little iodine, a pituitary problem, treatment for an overactive thyroid or thyroid cancer, severe acute illness, malnutrition or fasting, and certain medicines.
High free T4 can come from Graves disease, thyroiditis, toxic goiter or nodules, or taking too much thyroid hormone medicine. Rarely, and usually only in people with a thyroid problem, iodine contrast dye or a lot of iodine-rich food can raise it.
An abnormal result usually leads to at least one more test, such as TSH, free T3 or TPO antibodies. A comprehensive thyroid panel checks several of these at once.
Before the test
- Biotin (vitamin B7) can affect results. Tell your clinician if you take it and ask whether to pause it.
- Pregnancy, birth control pills, steroids, and kidney or liver disease can affect results.
- All medicines and supplements, including thyroid hormone, should be on the list you share. Don't stop any medicine without your prescriber.
See a clinician promptly if an abnormal free T4 comes with a racing or irregular heartbeat, unexplained weight loss, tremor, or neck swelling that makes swallowing or breathing hard. Call 911 for chest pain, fainting or trouble breathing.
Common questions
What is a normal free T4 level?
MedlinePlus lists about 0.8 to 1.9 ng/dL (10.3 to 24.5 pmol/L) as a typical lab range, but ranges vary by lab. Gabriel's optimal range, shown in the table on this page, is narrower.
Is free T4 or total T4 better?
Free T4 is usually preferred because it measures only the active, unbound hormone, and experts consider it more accurate. Total T4 includes the protein-bound hormone too.
What does high TSH and normal free T4 mean?
A high TSH most often means an underactive thyroid, according to the NIDDK. When TSH is abnormal, you usually need at least one more test, such as T4, T3 or thyroid antibodies, to find the cause.
Can biotin affect a free T4 test?
Yes. Biotin can affect free T4 results. Tell your clinician and ask whether to pause it before your blood draw.
Can free T4 be normal with an overactive thyroid?
Yes. Sometimes T4 is normal while T3 is high, so a clinician may add a T3 test when an overactive thyroid is suspected.
Sources
- Free T4 test MedlinePlus, U.S. National Library of Medicine
- Thyroxine (T4) Test MedlinePlus, U.S. National Library of Medicine
- Thyroid Tests National Institute of Diabetes and Digestive and Kidney Diseases
This guide is general information, not medical advice. It does not replace a visit with a licensed clinician. Gabriel membership is not health insurance.