Free T3 (free triiodothyronine)
Free T3 is the unbound share of the thyroid hormone T3. Published ranges sit near 2.1 to 4.4 pg/mL and depend on the assay. A US guideline reports limited use in hypothyroidism because levels often stay normal. Low free T3 went with more deaths in people with heart disease, and trials of T3 treatment after surgery did not change mortality.
Checked 2026-11-13
- Status
- A blood test ordered by a clinician and also sold direct to consumers by some US laboratories. FDA's device regulation for triiodothyronine assays, titled for total triiodothyronine, places them in class II and exempts them from premarket notification [1]. A US guideline states that T3 measurement, total or free, has limited utility in hypothyroidism because levels are often normal [2].
- Measures
- The triiodothyronine (T3) in blood that is not bound to protein. About 99.7% of T3 in blood is protein-bound [2].
- Also called
- free T3, FT3, fT3, free triiodothyronine, triiodothyronine free, T3 free, free T3 direct, direct free T3
- Tested for
- thyroid check, overactive thyroid, T3 conversion, fatigue, slow metabolism, weight gain, checking a thyroid medication dose
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Linked in research1
Reported risks0
No risks sourced yet.
Reference ranges
Each range is the one its source published, in pg/mL. Also reported in pmol/L. Where sources differ, each is shown.
US National Library of Medicine, MedlinePlus
Adults aged 20 and older · [3]
| Range | What the source calls it |
|---|---|
| below 2.1 pg/mL | No name given |
| 2.1 to 4.4 pg/mL | "normal" |
| above 4.4 pg/mL | No name given |
MedlinePlus prints the range as 210 to 440 pg/dL, or 3.2 to 6.8 pmol/L. Dividing pg/dL by 100 gives the pg/mL shown here. It states that normal values are age specific for people under 20 and that ranges vary slightly among laboratories. It names no assay [3].
American Board of Internal Medicine laboratory reference ranges, 2026
Adults, as a general laboratory reference range · [4]
| Range | What the source calls it |
|---|---|
| below 2.3 pg/mL | No name given |
| 2.3 to 4.2 pg/mL | "reference range" |
| above 4.2 pg/mL | No name given |
The table names no assay, age group or pregnancy status [4]. In an international comparison, 9 of 14 free T3 procedures read below the reference method by up to 30% and one read 22% above it, so a result and its range belong to the assay that produced them [5].
What has been shown to move it
Tested, and not shown to change it1
Thyroid function
Shown to raise TSH slightly, by 0.25 mIU/L, in pooled trials, with no change in free T3 or free T4.
Free T3 (0.027 pmol/L, 95% CI -0.052 to 0.107) and free T4 (-0.003 pmol/L, 95% CI -0.018 to 0.011) did not change.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- Soy raised thyroid stimulating hormone (WMD 0.248 mIU/L, 95% CI 0.001 to 0.494, I² 80.31%). Free T3 (0.027 pmol/L, 95% CI -0.052 to 0.107) and free T4 (-0.003 pmol/L, 95% CI -0.018 to 0.011) did not change. The authors call the clinical significance of the TSH rise unclear [16].
- Population
- Randomized trials in 18 articles [16].
- Sources
- [16]
- Caveats
- High heterogeneity, and the lower bound of the TSH interval is close to zero [16].
How it compares with the reference method
Mixed results1
Biotin supplements and the assay
One small study found 10 mg a day of biotin gave falsely high results in 5 of 8 biotin-based competitive immunoassays, across analytes that included free T3.
The numbers and sources
- Compared with
- The same person's result before taking biotin
- Effect
- After 7 days of 10 mg a day of biotin, interference appeared in 9 of 23 assays built on biotin and streptavidin (39%) and in none of 14 assays built without them. Five of 8 biotin-based competitive immunoassays read falsely high and 4 of 15 biotin-based sandwich immunoassays read falsely low. The 11 analytes included free T3, free T4 and TSH, measured on 4 diagnostic systems [11].
- Population
- 6 adults described as healthy, mean age 38 [11].
- Sources
- [11]
- Caveats
- Six people. The abstract does not list which free T3 assays were affected, and many assays showed no interference [11].
Did not match the reference method1
Immunoassays against the reference method
One comparison found 9 of 14 free T3 procedures read below the reference method, by as much as 30%, and one read 22% above it.
The numbers and sources
- Compared with
- Equilibrium dialysis with isotope dilution mass spectrometry
- Effect
- An international working group compared 13 immunoassays and 1 dialysis-based laboratory procedure with a candidate reference procedure. Four had a mean bias within 10%, 9 had negative biases of up to 30% and 1 was positively biased by 22%. Correlation with the reference procedure ranged from 0.88 to 0.30, and total imprecision from 2.4% to 10.0%. Recalibration to the reference procedure removed the assay-specific biases, while sample-related effects remained [5].
- Population
- A panel of single-donor sera measured with assays from 9 manufacturers [5].
- Sources
- [5]
- Caveats
- Published in 2010, so the assays may since have changed. The abstract names no assay [5].
What studies tie it to
Linked in research1
Death in people with heart disease
One cohort found more deaths within a year among heart patients with low free T3: 14.4% against 3% with normal free T3.
The numbers and sources
- Effect
- Among 573 consecutive patients with heart disease, 173 had a free T3 below 3.1 pmol/L. Over 1 year, deaths from any cause were 14.4% in that group and 3% in the group at 3.1 pmol/L or above, and cardiac deaths were 7.5% and 1.5%. In a Cox model free T3 was the strongest predictor of death from any cause (hazard ratio 3.582) [6].
- Population
- 573 consecutive cardiac patients at one center, followed for 1 year [6].
- Sources
- [6]
- Caveats
- One observational cohort of hospital patients. A US guideline states that T3 levels are low in severe illness without thyroid disease, because less T4 is converted to T3 [2], so low free T3 may mark the severity of illness.
Mixed results1
Heart function and death in trials that gave T3
Mixed results: trials giving T3 after heart surgery or in heart failure with low T3 improved measures of heart function, and deaths after surgery did not differ.
The numbers and sources
- Effect
- In a systematic review of 14 randomized trials of T3 after surgery, 13 of them cardiac surgery, intravenous T3 increased cardiac index after coronary bypass. Mortality was not significantly altered by high-dose intravenous T3 and could not be assessed for the other regimens [8]. In a meta-analysis of randomized trials in heart failure with low T3, thyroid hormone treatment raised free T3 and improved left ventricular ejection fraction (weighted mean difference 5.61, 95% CI 4.38 to 6.85) and cardiac output (0.65, 0.42 to 0.89). The abstract reports no data on deaths [9].
- Population
- 14 randomized trials of postoperative T3 in adults [8]. Randomized trials in heart failure with low T3 found in English and Chinese databases to November 2021 [9].
- Sources
- [8], [9]
- Caveats
- A marker that tracks an outcome is not always one whose correction changes it. The surgical trials were small and short, with T3 given for 6 to 24 hours in most. The heart failure analysis had high heterogeneity, with I-squared of 63% to 95% for its main outcomes [8][9].
Checked, no link found2
Death in older adults with normal thyroid tests
One cohort found no link between free T3 and death over 9 years in adults aged 65 and older whose thyroid tests were all in range.
The numbers and sources
- Effect
- Among 815 adults aged 65 and older with TSH, free T3 and free T4 all in the normal range, 181 died over 9 years. Neither free T3 nor free T4 quartile was associated with death from any cause. TSH in the lowest quartile was, with a hazard ratio of 2.22 (95% CI 1.19 to 4.22) against the highest quartile [7].
- Population
- 815 community-dwelling adults aged 65 and older in the Invecchiare in Chianti study in Italy [7].
- Sources
- [7]
- Caveats
- One observational cohort with 181 deaths, limited to people whose results were inside the reference ranges [7].
Symptoms in trials that added T3 to thyroxine for hypothyroidism
Not shown to change pain, mood, fatigue, quality of life, weight or cholesterol when T3 was added to thyroxine in 11 trials of adults with hypothyroidism.
The numbers and sources
- Effect
- A meta-analysis of 11 randomized trials found no difference between combined T4 and T3 treatment and T4 alone for bodily pain (standardized mean difference 0.00, 95% CI -0.34 to 0.35), depression (0.07, -0.20 to 0.34), anxiety (0.00, -0.12 to 0.11), fatigue (-0.12, -0.33 to 0.09) or quality of life (0.03, -0.09 to 0.15). Body weight and blood lipids did not differ, and adverse events did not differ between regimens [10].
- Population
- 1,216 adults with clinical hypothyroidism in 11 randomized trials [10].
- Sources
- [10]
- Caveats
- The trials compared treatments in people already treated for hypothyroidism, and the abstract does not report the free T3 levels reached. Searches ran to September 2005 [10].
Sources
- US Food and Drug Administration. 21 CFR 862.1710, Total triiodothyronine test system. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-21/section-862.1710
- Garber JR, Cobin RH, Gharib H, et al. Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocrine Practice. 2012. https://doi.org/10.4158/ep12280.gl
- US National Library of Medicine. T3 test. MedlinePlus Medical Encyclopedia. Reviewed January 25, 2026. https://medlineplus.gov/ency/article/003687.htm
- American Board of Internal Medicine. ABIM Laboratory Test Reference Ranges. January 2026. https://www.abim.org/Media/bfijryql/laboratory-reference-ranges.pdf
- Thienpont LM, Van Uytfanghe K, Beastall G, et al. Report of the IFCC Working Group for Standardization of Thyroid Function Tests; Part 2: Free Thyroxine and Free Triiodothyronine. Clinical Chemistry. 2010. https://doi.org/10.1373/clinchem.2009.140194
- Iervasi G, Pingitore A, Landi P, et al. Low-T3 Syndrome: A Strong Prognostic Predictor of Death in Patients With Heart Disease. Circulation. 2003. https://doi.org/10.1161/01.cir.0000048124.64204.3f
- Ceresini G, Marina M, Lauretani F, et al. Relationship Between Circulating Thyroid-Stimulating Hormone, Free Thyroxine, and Free Triiodothyronine Concentrations and 9-Year Mortality in Euthyroid Elderly Adults. Journal of the American Geriatrics Society. 2016. https://doi.org/10.1111/jgs.14029
- Kaptein EM, Sanchez A, Beale E, et al. Thyroid Hormone Therapy for Postoperative Nonthyroidal Illnesses: A Systematic Review and Synthesis. The Journal of Clinical Endocrinology and Metabolism. 2010. https://doi.org/10.1210/jc.2010-1052
- Shi C, Bao Y, Chen X, et al. The Effectiveness of Thyroid Hormone Replacement Therapy on Heart Failure and Low-Triiodothyronine Syndrome: An Updated Systematic Review and Meta-analysis of Randomized Controlled Trials. Endocrine Practice. 2022. https://doi.org/10.1016/j.eprac.2022.07.004
- Grozinsky-Glasberg S, Fraser A, Nahshoni E, et al. Thyroxine-Triiodothyronine Combination Therapy Versus Thyroxine Monotherapy for Clinical Hypothyroidism: Meta-Analysis of Randomized Controlled Trials. The Journal of Clinical Endocrinology and Metabolism. 2006. https://doi.org/10.1210/jc.2006-0448
- Li D, Radulescu A, Shrestha RT, et al. Association of Biotin Ingestion With Performance of Hormone and Nonhormone Assays in Healthy Adults. JAMA. 2017. https://doi.org/10.1001/jama.2017.13705