What is free T3?
T3 (triiodothyronine) is the thyroid hormone that does the work in cells. The thyroid mainly releases T4, which is converted to T3 by removing one iodine atom, mostly in the liver and in tissues where T3 acts, such as the brain. Like T4, T3 circulates almost entirely bound to transport proteins; free T3 is the unbound part that can act on cells.
T3 has a narrower role in testing than many people expect. The American Thyroid Association (ATA) says T3 is most useful when hyperthyroidism is suspected, because in some people with a low TSH only the T3 is raised. In hypothyroidism it is rarely helpful: T3 is the last test to become abnormal, and people can be severely hypothyroid with a normal T3. The ATA also notes that free T3 measurement is often not reliable.
Free T3 normal range
| Test | Range | In pmol/L or nmol/L | Source |
|---|---|---|---|
| Free T3, adults | 2.1 to 4.4 pg/mL (210 to 440 pg/dL) | 3.2 to 6.8 pmol/L | MedlinePlus |
| Free T3, adults 20 and over | 2.5 to 4.3 pg/mL | 3.8 to 6.6 pmol/L | ARUP |
| Free T3, 12 to 19 years | 2.3 to 5.0 pg/mL | 3.5 to 7.7 pmol/L | ARUP |
| Total T3, adults | 78 to 158 ng/dL | 1.2 to 2.4 nmol/L | MedlinePlus |
To convert free T3 from pg/mL to pmol/L, multiply by 1.536 (the pmol/L figures for ARUP ranges are our conversions). Watch the units on your report: MedlinePlus uses pg/dL, which is 100 times the pg/mL number. Women and men share one adult range, and people under 20 have age-specific ranges.
MedlinePlus and ARUP Laboratories. MedlinePlus writes its range as 210 to 440 pg/dL.
Free T3 or total T3?
Total T3 measures bound and free hormone together, so like total T4 it moves with the transport proteins that pregnancy and birth control pills increase. MedlinePlus notes a high T3 can occur in pregnancy or with birth control pills and estrogen for this reason. Free T3 is the unbound fraction, and the ATA considers free T3 measurement often unreliable, so if a T3 test is needed, ask your doctor which one they want.
What high free T3 means
MedlinePlus lists these causes of a higher-than-normal T3:
- An overactive thyroid, for example Graves' disease
- T3 thyrotoxicosis, a rare form where only T3 is high
- Toxic nodular goiter
- Taking thyroid medicine or certain supplements (common)
- Liver disease
Pregnancy, birth control pills and estrogen can also raise T3 readings. A high free T3 with a low TSH should go to your doctor.
Worked example: free T3 5.1 with a low TSH
Say a 44-year-old man gets a free T3 of 5.1 pg/mL, a TSH of 0.05 mIU/L and a free T4 of 1.6 ng/dL. Free T3 is above both the MedlinePlus (4.4) and ARUP (4.3) upper limits, TSH is far below the 0.4 mIU/L floor, and free T4 is still inside its range.
This is the case where the ATA says T3 earns its place: in some people with a low TSH, only T3 is raised. MedlinePlus lists Graves' disease, toxic nodular goiter and T3 thyrotoxicosis among the causes, and taking thyroid medicine or certain supplements is a common one, so he should list everything he takes.
Questions for the doctor: Could this be an overactive thyroid even though free T4 is normal? Should we test thyroid stimulating immunoglobulin (TSI), the antibody the ATA links to Graves' disease? Could any supplement I take contain thyroid hormone?
What low free T3 means
Serious illness is a common reason for a low T3. MedlinePlus lists short-term or long-term illness, thyroiditis (Hashimoto's disease is the most common type), starvation and an underactive thyroid. Because T3 is the last marker to fall in hypothyroidism, a low free T3 on its own, with normal TSH and free T4, rarely changes treatment. Selenium is needed to convert T4 to T3, but MedlinePlus notes it is not clear that selenium deficiency actually lowers T3 levels in people.
What can change free T3
- Medicines that can raise T3: birth control pills, estrogens, methadone and some herbal remedies.
- Medicines that can lower T3: amiodarone, anabolic steroids and androgens, antithyroid drugs (methimazole, propylthiouracil), lithium, phenytoin and propranolol.
- Biotin: can distort thyroid immunoassays; the ATA advises stopping it 2 days before testing.
- Illness and calorie restriction both lower T3.
Never stop or change a prescribed medicine to improve a test result without your doctor.
How to test free T3
- Stop biotin for 2 days first.
- Test when you are well, not during or just after an acute illness.
- Always read free T3 with TSH and free T4, and at the same lab each time.
What about the free T3:free T4 ratio?
Some panels, including SiPhox's, report a free T3:free T4 ratio. The ATA's guide to thyroid function tests does not use it and we found no guideline that sets a normal range for it, so treat it as background information rather than something to act on.
Common questions about free T3
Is free T3 worth testing on its own?
Rarely. The ATA says T3 tests are most useful to diagnose hyperthyroidism or judge how severe it is, and that T3 is the last test to become abnormal in hypothyroidism. Order it with TSH and free T4 and read it through them.
What can make free T3 look off?
Biotin can distort the assay (stop it 2 days before). Acute illness and calorie restriction lower T3, so a low value drawn while you are sick is worth repeating after you recover. Birth control pills and estrogen raise T3 readings, mainly total T3, because they increase the proteins that carry it.
Which antibody tests go with a high free T3?
MedlinePlus lists thyroid stimulating immunoglobulin (TSI) and thyrotropin receptor antibodies (TRAb) as markers that may indicate Graves' disease, while TPO antibodies and thyroglobulin antibodies point mainly to Hashimoto's. SiPhox panels do not include TSI or TRAb, so those go through your doctor.
Which at-home tests include free T3
SiPhox Health measures free T3 on two panels: Thyroid Focus, included in the base $124 membership price, and Ultimate 360, which costs $125 more per test. GLP Optimization includes TSH and free T4 but not free T3, and Longevity Essentials and Heart & Metabolic include TSH only.
Thyroid Focus is the better value for a thyroid question: it adds TPO antibodies, thyroglobulin antibodies and ferritin for the same price as any core panel. See our at-home thyroid test guide for other options.
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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What is a normal free T3 level?
About 2.1 to 4.4 pg/mL (3.2 to 6.8 pmol/L) on the MedlinePlus range, or 2.5 to 4.3 pg/mL at ARUP for adults. MedlinePlus writes the range in pg/dL (210 to 440), so check the unit on your report.
What is the difference between free T3 and free T4?
T4 is the main hormone the thyroid releases; T3 is the more active form, mostly made from T4 in tissues. Free T4 is the better test of thyroid function, while free T3 mainly helps when an overactive thyroid is suspected.
Can free T3 be low with normal TSH?
Yes, especially during illness or calorie restriction. With normal TSH and free T4, a low free T3 on its own rarely points to thyroid disease, but discuss it with your doctor if you have symptoms.
Do I need free T3 if I have hypothyroidism?
Usually not. The American Thyroid Association says T3 is rarely helpful in hypothyroidism because it is the last test to become abnormal. TSH and free T4 guide diagnosis and dosing.
Sources
- MedlinePlus Medical Encyclopedia: T3 test
- MedlinePlus Lab Tests: Triiodothyronine (T3) tests
- ARUP Laboratories Test Directory: Triiodothyronine, free (free T3)
- American Thyroid Association: Thyroid function tests
- MedlinePlus Lab Tests: Thyroid antibodies
- SiPhox Health: panel comparison (checked 29 Sep 2026)
- SiPhox Health: Thyroid Focus panel