Free T3 levels: what's normal, high and low
The active, unbound form of T3 — the most metabolically active thyroid hormone.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
The short answer
Free T3 is measured with a standard blood draw; no fasting is needed. A typical reference range is 2.00 – 4.40 pg/mL. At TestWell it's $30.99 all-in as Free T3 (Triiodothyronine), results in 1–3 business days, drawn at Quest or Labcorp. An out-of-range result is a prompt to look closer, not a diagnosis.
- Units
- pg/mL
- Sample
- Blood draw
- Fasting
- Not required
- Results
- 1–3 business days
- Price
- $30.99
- Labs
- Quest or Labcorp · BioReference in NY/NJ
Free T3 Reference Range
These are the typical adult reference ranges used by major US labs. Your report will include the specific range used by the lab that processed your sample.
| Population | Range |
|---|---|
| age 18+ | 2.00 – 4.40 pg/mL |
Reference ranges are educational. Always interpret your result with the specific range printed on your lab report and discuss anything outside the range with a clinician.
Have a result in hand? analyze it with your AI assistant using these reviewed ranges, or run it through our explainer. Tools: look up another marker's range · convert lab units
What does a high Free T3 mean?
High Free T3 typically accompanies hyperthyroidism, and in some cases (T3 toxicosis) it is the only hormone elevated while Free T4 is still normal. It also rises on liothyronine or desiccated thyroid, especially if drawn within hours of a dose, and biotin can distort the assay. It is read together with TSH.
Common causes of a high Free T3
- Hyperthyroidism, including early Graves' disease and T3 toxicosis
- T3-containing medication (liothyronine, desiccated thyroid), especially post-dose
- Biotin interference
- Rarely, thyroid hormone resistance
Ordered from most to least common.
Symptoms that can go with it
- Palpitations, heat intolerance, sweating
- Anxiety, tremor, insomnia
- Weight loss
What clinicians usually check next
- TSH and Free T4 to define the pattern
- Review T3-containing medication timing
- Antibodies or uptake scan as directed
Follow-up tests at TestWell: TSH (Thyroid Stimulating Hormone) ($26.99), TSH + Free T4 ($27.99), Free T4 (Thyroxine) ($23.99), T3 Uptake ($22.99), T3, Total (Triiodothyronine) ($24.99).
Can Free T3 be high when TSH and Free T4 are normal?
Occasionally — early hyperthyroidism, medication timing or assay interference. A repeat pre-dose, off biotin, usually clarifies it.
Educational only — not a diagnosis. A single result outside the range is a prompt to look closer with a clinician.
What does a low Free T3 mean?
Low Free T3 is less specific than a low Free T4. It occurs in hypothyroidism, but also very commonly during illness, fasting or calorie restriction, when the body slows conversion of T4 to T3 — a normal adaptation, not a thyroid disease. Some medications (beta-blockers, glucocorticoids, amiodarone) lower it too. It is interpreted with TSH and Free T4.
Common causes of a low Free T3
- Non-thyroidal illness, fasting, very low-calorie dieting
- Hypothyroidism (usually with a high TSH and low Free T4)
- Medications: glucocorticoids, propranolol, amiodarone, some contrast agents
- Older age (mild decline is normal)
Ordered from most to least common.
Symptoms that can go with it
- Often none from the low T3 itself
- Fatigue, cold intolerance if hypothyroid
What clinicians usually check next
- Interpret with TSH and Free T4 — an isolated low Free T3 with both normal rarely needs treatment
- Retest when well and eating normally
- Review medications
Follow-up tests at TestWell: TSH (Thyroid Stimulating Hormone) ($26.99), TSH + Free T4 ($27.99), Free T4 (Thyroxine) ($23.99), T3 Uptake ($22.99), T3, Total (Triiodothyronine) ($24.99).
My Free T3 is low but everything else is normal. Do I need T3 medication?
Usually not. Isolated low Free T3 most often reflects illness, dieting or medications; guidelines don't support treating it in isolation. Discuss with your clinician.
Educational only — not a diagnosis. A single result outside the range is a prompt to look closer with a clinician.
How do you get a Free T3 test?
Order online, a licensed physician reviews the requisition, and you walk into a nearby lab for the draw. Free T3 is available on its own or inside these panels — panels cost less per marker when you want the fuller picture.
| Option | Includes | Price |
|---|---|---|
| Free T3 (Triiodothyronine) | This marker only | $30.99 |
| Essential Thyroid Panel | 3 tests | $49.99 |
| Hashimoto's Thyroid Panel | 3 tests | $97.99 |
| Complete Thyroid Panel | 4 tests | $109.99 |
| Advanced Thyroid Panel | 7 tests | $129.99 |
Quest vs. Labcorp for Free T3: which lab should you pick?
Both are CLIA-certified national labs and the result is clinically equivalent, so pick whichever location is closer. Your report will print that lab's own reference range, which can differ slightly from the table above.
| Lab | Available | Price |
|---|---|---|
| Quest Diagnostics | Yes | $30.99 |
| Labcorp | Yes | $30.99 |
| BioReference (NY & NJ) | Yes | $32.99 |
Need help choosing a location? Use the lab finder to see draw sites near your ZIP.
Related Thyroid biomarkers
Markers commonly tested alongside Free T3.
Frequently asked
What does a high Free T3 mean?
High Free T3 typically accompanies hyperthyroidism, and in some cases (T3 toxicosis) it is the only hormone elevated while Free T4 is still normal. It also rises on liothyronine or desiccated thyroid, especially if drawn within hours of a dose, and biotin can distort the assay. It is read together with TSH.
Can Free T3 be high when TSH and Free T4 are normal?
Occasionally — early hyperthyroidism, medication timing or assay interference. A repeat pre-dose, off biotin, usually clarifies it.
What does a low Free T3 mean?
Low Free T3 is less specific than a low Free T4. It occurs in hypothyroidism, but also very commonly during illness, fasting or calorie restriction, when the body slows conversion of T4 to T3 — a normal adaptation, not a thyroid disease. Some medications (beta-blockers, glucocorticoids, amiodarone) lower it too. It is interpreted with TSH and Free T4.
My Free T3 is low but everything else is normal. Do I need T3 medication?
Usually not. Isolated low Free T3 most often reflects illness, dieting or medications; guidelines don't support treating it in isolation. Discuss with your clinician.
Do I need a doctor's order to test my free t3?
No. TestWell's licensed physician network reviews and approves your order — usually within minutes. You can place an order online without an in-person visit or a referral from your own doctor.
How long does it take to get Free T3 results?
Most Free T3 results are available in 1–3 business days after your specimen arrives at the lab. You'll get an email the moment your results are ready, with everything viewable in your TestWell portal.
How much does a Free T3 test cost?
Our Free T3 (Triiodothyronine) starts at $30.99. That's the all-in price — no copays, no insurance hoops, no surprise bills. HSA and FSA cards are accepted.
Do I need to fast before my Free T3 test?
Fasting is not required for the Free T3 test. You can have your blood drawn any time of day.
Which lab will process my Free T3 test?
Your test will be processed by Quest Diagnostics, Labcorp, or BioReference depending on which is closest to you and which lab carries the test. All three are CLIA-certified and trusted by hospitals nationwide. You'll choose your lab during checkout from 4,000+ locations.
What can throw off a Free T3 result?
Biotin supplements, levothyroxine timing and recent illness can change a Free T3 result without anything being wrong. Biotin supplements can falsely raise Free T3 — stop biotin ≥72 hours before the draw. Levothyroxine timing can raise Free T3 — draw before the dose. Recent illness can shift Free T3 — wait 4–6 weeks after significant illness before judging thyroid. Each is explained on our physician-reviewed "Before your test" pages; when one applies, remove it and retest.
Ready to test your free t3?
Order in two minutes, get physician-approved, and visit a lab the same day. Starts at $30.99.