What is free T4?
T4 (thyroxine) is the main thyroid hormone circulating in your blood. Most of it is bound to carrier proteins; the small unbound part, free T4, is what can enter tissues and act on them. The body converts T4 into the more active T3 as needed.
A total T4 test measures bound and free hormone together, so it shifts whenever binding proteins change, for example with estrogen, birth control pills or pregnancy. The American Thyroid Association (ATA) says free T4 reflects thyroid function more accurately, which is why most doctors order free T4 alongside TSH.
Free T4 normal range
| Group | ng/dL | pmol/L | Source |
|---|---|---|---|
| Adults (typical range) | 0.8 to 1.9 | 10.3 to 24.5 | MedlinePlus |
| Adults 20 and over | 0.9 to 1.7 | 11.6 to 21.9 | ARUP |
| 12 to 19 years | 0.9 to 1.6 | 11.6 to 20.6 | ARUP |
| Pregnancy, 1st trimester | 0.9 to 1.4 | 11.6 to 18.0 | ARUP |
| Pregnancy, 2nd trimester | 0.7 to 1.3 | 9.0 to 16.7 | ARUP |
To convert ng/dL to pmol/L, multiply by 12.87 (the pmol/L figures for ARUP ranges are our conversions). MedlinePlus and ARUP give one adult range for women and men. The range is built from a large population, so MedlinePlus notes you can have thyroid symptoms with a free T4 inside it; TSH helps settle the question.
MedlinePlus and ARUP Laboratories reference intervals.
Free T4, total T4 and the free T4 index
Your report may show one of three T4 numbers. Free T4 is measured directly. Total T4 includes the bound hormone and runs high whenever binding proteins rise, as they do in pregnancy and with birth control pills, even when thyroid function is normal. The free T4 index (FTI) is a calculated estimate of free T4 from a total T4 result. If you are pregnant or take estrogen, the ATA suggests asking for TSH and free T4 rather than total T4.
What high free T4 means
A high free T4 usually means an overactive thyroid (hyperthyroidism). MedlinePlus lists these causes:
- Graves' disease
- Taking too much thyroid hormone medicine
- Thyroiditis (inflammation of the thyroid)
- Toxic goiter or toxic thyroid nodules
- Rarely, iodine contrast dye or a very high iodine intake when the thyroid is already abnormal, or some tumors of the testes or ovaries
With hyperthyroidism, TSH is usually low at the same time. If your free T4 is high, see your doctor.
What low free T4 means
A low free T4 usually means an underactive thyroid (hypothyroidism), including Hashimoto's disease, which is picked up by TPO antibodies. MedlinePlus also lists severe acute illness, malnutrition or fasting, and certain medicines.
Low free T4 with normal TSH
Normally a low free T4 drives TSH up. If free T4 is low and TSH is normal or low, the problem may sit in the pituitary rather than the thyroid (secondary hypothyroidism), or the result may reflect a serious illness or a lab interference such as biotin. This pattern needs a doctor's review and usually a repeat test.
| Free T4 | TSH | Usual meaning |
|---|---|---|
| Low | High | Hypothyroidism |
| Normal | High | Mild (subclinical) hypothyroidism |
| High | Low | Hyperthyroidism |
| Low | Normal or low | Possible pituitary problem, illness or interference |
Worked example: free T4 0.7 with TSH 9.5
Say a 52-year-old woman gets a free T4 of 0.7 ng/dL and a TSH of 9.5 mIU/L. Free T4 is below both the MedlinePlus (0.8) and ARUP (0.9) lower limits, and TSH is about double the MedlinePlus upper limit of 4.8. On the table above, low free T4 with high TSH is the pattern of hypothyroidism.
Hashimoto's disease is the most common cause of an underactive thyroid, so TPO antibodies are the logical next test. Biotin can distort both numbers, so she should confirm she stopped it 2 days before the draw.
Questions for the doctor: Does this confirm hypothyroidism, or should we repeat both tests first? Should we check TPO antibodies? If I start levothyroxine, when do we retest? The ATA says blood is checked 6 to 8 weeks after starting or changing a dose.
What can change free T4
- Biotin. Can distort thyroid test results. The ATA advises stopping it for 2 days before testing.
- Pregnancy and estrogen. Both change binding proteins and free T4 ranges.
- Liver and kidney disease, and rare inherited changes in the proteins that carry T4.
- Thyroid medicine. Levothyroxine is T4, so taking it changes the result. Your doctor decides the dose.
- Iodine. Contrast dye and very high intake can affect an already abnormal thyroid.
How to test free T4
- Stop biotin supplements 2 days before (ATA).
- Test free T4 with TSH, since neither tells the whole story alone.
- Tell whoever orders the test if you are pregnant or taking estrogen or thyroid medicine.
- No fasting is needed for free T4 itself.
Common questions about free T4
How often should free T4 be checked?
If you take thyroid hormone, the ATA says blood is checked 6 to 8 weeks after starting or changing the dose, and MedlinePlus says people on thyroid medicine have regular TSH tests. The ATA lists antacids, calcium or iron supplements, biotin, estrogen and some seizure medicines as things that affect thyroid pills, so a change in any of them is a reason to ask about retesting.
What should be tested alongside free T4?
TSH every time. Add TPO antibodies when TSH is high, to look for Hashimoto's. Free T3 helps mainly when TSH is low, because the ATA says in some people with an overactive thyroid only T3 is raised. If you are pregnant, ask for trimester ranges: ARUP's second-trimester range (0.7 to 1.3 ng/dL) sits below the adult range.
Which at-home tests include free T4
SiPhox Health measures free T4 on three panels:
- Thyroid Focus (included in the base price): TSH, free T4, free T3, TPO and thyroglobulin antibodies, plus the free T3:free T4 and TSH:T4 ratios.
- Ultimate 360 (+$125 a test): the full thyroid set plus 55 other markers.
- GLP Optimization (+$75 a test): TSH, free T4 and the TSH:T4 ratio.
Longevity Essentials and Heart & Metabolic include TSH but not free T4, and Hormone Focus has neither. Compare kits in our at-home thyroid test guide, or read the SiPhox review.
A note on home samples: SiPhox's partner lab tests capillary blood collected from the upper arm. SiPhox says its results show a 95% to 99% correlation with venous draws in its own quarterly comparisons against Labcorp and Quest, but it has not published a study of the EasyDraw device. If a result would change a treatment decision, confirm it with a venous test through your doctor.
Thyroid Focus
A full thyroid workup including both thyroid antibodies, plus core lipids.
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Ultimate 360
Everything on the four core panels plus iron studies, B12 and folate, GGT and PSA.
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GLP Optimization
Built for people on GLP-1 medication: metabolic, liver, kidney, nutrient and iron markers.
Check priceFrequently asked questions
What is a normal free T4 level?
About 0.8 to 1.9 ng/dL (10.3 to 24.5 pmol/L) on the MedlinePlus range, or 0.9 to 1.7 ng/dL at ARUP for adults. Use the range printed on your own report.
What does low free T4 with normal TSH mean?
It can point to a pituitary problem (secondary hypothyroidism), a serious illness or a lab interference such as biotin. It is an unusual pattern that needs your doctor to review it, usually with a repeat test.
Is free T4 better than total T4?
Usually yes. Total T4 changes with binding proteins, which estrogen and pregnancy raise. The American Thyroid Association says free T4 more accurately reflects thyroid function.
Do I need to fast for a free T4 test?
No. Free T4 does not need fasting. Do stop biotin for 2 days first, and follow any fasting rules for other tests on the same draw.
Sources
- MedlinePlus Medical Encyclopedia: Free T4 test
- MedlinePlus Lab Tests: Thyroxine (T4) test
- ARUP Laboratories Test Directory: Thyroxine, free (free T4)
- American Thyroid Association: Thyroid function tests
- American Thyroid Association: Hypothyroidism (underactive thyroid)
- MedlinePlus Lab Tests: TSH (thyroid stimulating hormone) test
- SiPhox Health: panel comparison (checked 29 Sep 2026)
- SiPhox Health: Thyroid Focus panel
- SiPhox Health: GLP Optimization panel