Biomarkers · Chemical

Triiodothyronine (T3), Free

A triiodothyronine (T3) test measures the level of T3 in a sample of your blood. It's used to help diagnose thyroid disease.

Triiodothyronine (T3) Tests, MedlinePlus, National Library of Medicine

Free T3 is the active form of T3 in the blood, the portion that enters tissues where it is needed. Thyroid hormones work together to control how the body uses energy.

Primary unit: pg/mL
LOINC codes: 3051-0
Tests that measure Triiodothyronine (T3), Free: Labcorp 1, Quest Diagnostics 1

See why clinicians order the test

Clinicians order T3 testing to help diagnose hyperthyroidism, a condition in which the thyroid gland makes too much thyroid hormone.
Condition: Hyperthyroidism
When TSH or T4 results look abnormal or unclear, clinicians may use a T3 test to confirm or rule out hyperthyroidism.
Condition: Hyperthyroidism
Clinicians use T3 testing to monitor treatment for thyroid disease, including both hyperthyroidism and hypothyroidism.
Goal: Thyroid function
Clinicians may use T3 testing when looking for pituitary disorders, since abnormal T3 levels can signal a pituitary problem.
Studies of relative energy deficiency in sport (REDs) use T3 among their most frequent blood markers.
Goal: Training recovery and energy availability

Read with companion markers

T4, Free (Direct)
Providers usually measure free T4 when reading thyroid results, since T4, like T3, can be measured as free or total.
TSH
Clinicians usually compare T3 results with TSH and T4 results to understand what the T3 result indicates.

Choose the right version

Triiodothyronine (T3)Total
Total T3 measures bound and free T3 together, and MedlinePlus reports that experts consider it the more accurate T3 test.

Check what can shift results

Physiology
- In exercising women, T3 and free T3 fell once energy availability dropped below a threshold, while free T4 rose.
Condition
- Acute illness can lower plasma T3 concentrations without a matching rise in TSH.
Medicine
- Certain medicines and supplements can change T3 test results apart from the thyroid condition being tested.
Supplement
- Biotin interference can falsely raise FT3 and FT4 while falsely lowering TSH, producing a pattern that mimics hyperthyroidism.

Prepare for the test

The T3 blood test needs no special preparation beforehand, according to MedlinePlus.
MedlinePlus advises against stopping any medicines before the test unless the provider gives that instruction.

Each statement links to the source we wrote it from. Last updated October 19, 2026.

Ranges

Each lab publishes a reference range for the population groups it tests, usually the middle 95% of results. These intervals describe that population, not an optimal or healthy target. How ranges differ

Labcorp, Triiodothyronine (T3), Free 010389

Population	Interval
Age	Range (pg/mL)
0 to 3 d	2.0−7.9
4 to 30 d	2.0−5.2
31 d to 12 m	1.6−6.4
13 m to 5 y	2.0−6.0
6 to 10 y	2.7−5.2
11 to 19 y	2.3−5.0
>19 y	2.0−4.4

Labcorp test catalog, 010389

Tests

Check the catalog code, specimen, method and preparation for every test that measures Triiodothyronine (T3), Free. Select up to five tests to compare them side by side.

Labcorp 1 test

Single-analyte tests

Triiodothyronine (T3), Free 010389
Serum · Fasting not stated · Turnaround 1 - 2 days
Method and collection
Method
Electrochemiluminescence immunoassay (ECLIA) This assay is performed on the Roche cobas instrument in multiple laboratories throughout Labcorp.
Collection
If a red-top tube or plasma is used, transfer separated serum or plasma to a plastic transport tube.
Labcorp catalog page

Quest Diagnostics 1 test

Single-analyte tests

T3, Free 34429
Serum · Fasting not stated · Turnaround Set up: Sun-Fri; Report available: 3 days
Method and collection
Method
Immunoassay (IA)
Reference notes
<1 Month Not established 1-23 Months 3.3-5.2 pg/mL 2-12 Years 3.3-4.8 pg/mL 13-20 Years 3.0-4.7 pg/mL >20 Years 2.3-4.2 pg/mL
Quest Diagnostics catalog page

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Goals and conditions

We link Triiodothyronine (T3), Free to these goals and conditions from named guidelines and studies.

Thyroid functionGoal, companion marker
NICE measures free tri-iodothyronine with free thyroxine when TSH is below the reference range.
Thyroid disease: assessment and management (NG145), National Institute for Health and Care Excellence, 2019
Training recovery and energy availabilityGoal, primary marker
A 2024 review of REDs diagnosis found T3 among the biomarkers studies use most often, alongside bone density and body composition.
A literature review of biomarkers used for diagnosis of relative energy deficiency in sport, Frontiers in Sports and Active Living, 2024
HyperthyroidismCondition, ICD-10 E05.90, companion marker
NICE measures free tri-iodothyronine with free thyroxine when TSH is below the reference range.
Thyroid disease: assessment and management (NG145), National Institute for Health and Care Excellence, 2019

Sources

Check the guidelines, studies and lab catalogs behind every range and link on this page. Open a source to see each claim it backs.

Clinical guidelines

Thyroid disease: assessment and management (NG145)
National Institute for Health and Care Excellence, 2019
- Linked to Hyperthyroidism (companion)
  NICE measures free tri-iodothyronine with free thyroxine when TSH is below the reference range.
- Linked to Thyroid function (companion)
  NICE measures free tri-iodothyronine with free thyroxine when TSH is below the reference range.
Open source

Studies

A literature review of biomarkers used for diagnosis of relative energy deficiency in sport
Frontiers in Sports and Active Living, 2024
- Reasons to test
  Studies of relative energy deficiency in sport (REDs) use T3 among their most frequent blood markers.
  “The most frequently utilized biomarkers included BMD, anthropometric parameters (e.g., body mass index, body mass, and fat mass), and the triiodothyronine (T3) concentration.”
- Linked to Training recovery and energy availability (primary)
  A 2024 review of REDs diagnosis found T3 among the biomarkers studies use most often, alongside bone density and body composition.
Open source PubMed 39070233
Sense and nonsense concerning biotin interference in laboratory tests
Acta clinica Belgica, 2022
- What can shift results
  Biotin interference can falsely raise FT3 and FT4 while falsely lowering TSH, producing a pattern that mimics hyperthyroidism.
  “Parameters vary in their susceptibility: a combination of false positives and negatives mimicking a coherent profile is dangerous, e.g. combining falsely lowered TSH with falsely elevated FT4/FT3 mimicking hyperthyreosis.”
Open source PubMed 32567529
Nonthyroidal Illness Syndrome Across the Ages
Journal of the Endocrine Society, 2019
- What can shift results
  Acute illness can lower plasma T3 concentrations without a matching rise in TSH.
  “In conditions of acute illness, patients present with reduced plasma T3 concentrations without a concomitant rise in TSH.”
Open source PubMed 31745528
Induction of low-T3 syndrome in exercising women occurs at a threshold of energy availability
The American Journal of Physiology, 1994
- What can shift results
  In exercising women, T3 and free T3 fell once energy availability dropped below a threshold, while free T4 rose.
  “Repeated-measures one-way analysis of variance followed by one-sided, two-sample post hoc Fischer's least significant difference tests of changes by treatment day 4 revealed that reductions in T3 (16%, P < 0.00001) and fT3 (9%, P < 0.01) occurred abruptly between 19.0 and 25.0 kcal.kg LBM-1.day-1 and that increases in fT4 (11%, P < 0.05) and rT3 (22%, P < 0.01) occurred abruptly between 10.8 and 19.0 kcal.kg LBM-1.day-1.”
Open source PubMed 8160876

Lab catalogs

Labcorp test menu
Labcorp
- Test catalog: 1 tests, with codes, specimens, methods and reference intervals
Open source
Quest Diagnostics test menu
Quest Diagnostics
- Test catalog: 1 tests, with codes, specimens, methods and reference intervals
Open source

Reference sources

Triiodothyronine (T3) Tests
MedlinePlus, National Library of Medicine
- Before the test
  MedlinePlus advises against stopping any medicines before the test unless the provider gives that instruction.
  “But don't stop taking any medicines unless your provider tells you to.”
- Before the test
  The T3 blood test needs no special preparation beforehand, according to MedlinePlus.
  “You don't need any special preparations for a T3 blood test.”
- Read together with
  Providers usually measure free T4 when reading thyroid results, since T4, like T3, can be measured as free or total.
  “Like T3, T4 tests may measure either free or total T4, but providers usually measure free T4.”
- Read together with
  Clinicians usually compare T3 results with TSH and T4 results to understand what the T3 result indicates.
  “To understand the results of your T3 test, your provider will usually compare them with the results of your TSH and T4 tests.”
- Reasons to test
  Clinicians order T3 testing to help diagnose hyperthyroidism, a condition in which the thyroid gland makes too much thyroid hormone.
  “Diagnose hyperthyroidism, a condition in which your thyroid gland makes too much thyroid hormone.”
- Reasons to test
  When TSH or T4 results look abnormal or unclear, clinicians may use a T3 test to confirm or rule out hyperthyroidism.
  “If your test results are abnormal or unclear, your provider may use a T3 test to confirm or rule out hyperthyroidism.”
- Reasons to test
  Clinicians use T3 testing to monitor treatment for thyroid disease, including both hyperthyroidism and hypothyroidism.
  “Monitor treatment for thyroid disease, including hyperthyroidism and hypothyroidism, which is when the thyroid gland makes to little thyroid hormone.”
- Reasons to test
  Clinicians may use T3 testing when looking for pituitary disorders, since abnormal T3 levels can signal a pituitary problem.
  “So, abnormal T3 levels may be a sign of a pituitary problem.”
- Versions of the test
  Total T3 measures bound and free T3 together, and MedlinePlus reports that experts consider it the more accurate T3 test.
  “Medical experts think that this test is the more accurate way to measure T3.”
- What can shift results
  Certain medicines and supplements can change T3 test results apart from the thyroid condition being tested.
  “Certain medicines and supplements can affect your test results.”
- What it reflects
  Thyroid hormones work together to control how the body uses energy.
  “Thyroid hormones work together to control the way your body uses energy.”
- What it reflects
  Free T3 is the active form of T3 in the blood, the portion that enters tissues where it is needed.
  “Free T3 is the active form that enters your tissues where it's needed.”
Open source
Retrieved October 19, 2026
Triiodothyronine (T3) Tests
MedlinePlus, National Library of Medicine
- The description at the top of this page
Open source

Codes and registries

LOINC 3051-0
Regenstrief Institute codes that identify the same result across labs.
Search ClinicalTrials.gov for Triiodothyronine (T3), Free
Registered studies that measure Triiodothyronine (T3), Free, on the U.S. National Library of Medicine registry.

Verify these ranges and sources

We compile this page from named laboratory catalogs, guidelines and studies. Labs calculate reference intervals from the people they test, for one assay method and one population group, so an interval describes that population rather than an optimal target.

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