Total T3 (Triiodothyronine)
Measures all the T3 in your blood, the more active thyroid hormone, including the part attached to carrier proteins.
AI-drafted. Awaiting verificationAlso called TT3LOINC 3053-6
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What it is
T3 is the most active thyroid hormone. Your thyroid makes some, and the rest is made when your body converts T4 into T3. Almost all T3 travels attached to carrier proteins, and less than 1% is free. Total T3 counts the attached and the free parts together. It is not the same as free T3, and it is not the same as T3 uptake, which is a different test.
Why it matters
Total T3 is mostly used when someone has signs of an overactive thyroid. It can show a pattern sometimes called T3 toxicosis, where T3 is high while T4 is still in the usual range. Labcorp notes that in about 5% of overactive thyroid cases only T3 is raised. Mayo's follow-up testing path adds total T3 when TSH is very low and free T4 is in the usual range. It is not a helpful test for an underactive thyroid. Serious illness and fasting also lower T3, so a low value does not always point to the thyroid.
If it’s low
A lower total T3 is common during illness and fasting and does not always mean a thyroid problem. Total T3 is not a good test for an underactive thyroid.
Common causes
- Serious or long-term illness, when the body lowers T3 on its own
- Fasting, a very low calorie intake or poor nutrition
- Low carrier protein, including an inherited TBG shortage
- An underactive thyroid, although total T3 can stay in the usual range, so it is not used to check for this
What can help
- Ask whether a recent illness or fasting could explain a low value
- Ask whether TSH and free T4 are enough, or whether a repeat once you are well would help
If it’s high
A higher total T3 is linked to an overactive thyroid. It can also be higher when carrier protein rises, for example in pregnancy, or soon after taking T3 medicine.
Common causes
- Pregnancy or estrogen-containing medicines, which raise carrier protein and total T3 (by about 50% in pregnancy)
- Taking T3 (liothyronine) or thyroid extract, especially when blood is drawn soon after a dose
- Inherited extra carrier protein (TBG), which raises total T3 without a thyroid problem
- Graves' disease, an autoimmune cause of an overactive thyroid
- A single overactive thyroid nodule, or several nodules making extra hormone
What can help
- Ask how your result fits with your TSH, free T4 and free T3
- Tell your clinician about thyroid medicine, birth control, hormone therapy or pregnancy
- If you take thyroid medicine, ask how to time your dose around the blood draw
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
See Total T3 by age and sex, with the U.S. survey numbers where they exist.
What can affect your result
- Pregnancy: total T3 rises by about 50% because estrogen raises carrier proteins
- Estrogen from birth control or hormone therapy, which raises carrier protein
- Serious or long-term illness and fasting, which lower T3
- Timing of liothyronine or thyroid extract doses, with peaks soon after a dose
- Age: lab ranges differ for children and teens
- Biotin supplements, on some immunoassays
- Test method: immunoassay and mass spectrometry results can differ
- High-dose biotin can interfere with some immunoassays. Labcorp asks patients to stop biotin at least 72 hours before the draw; check with your clinician or lab first.
- Fasting lowers T3, and so do serious or long-term illnesses and poor nutrition (Labcorp).
- Changes in carrier proteins, such as those from pregnancy, estrogen-containing birth control or inherited TBG differences, move total T3 without a change in the thyroid (Labcorp).
- If you take liothyronine or thyroid extract, T3 peaks soon after a dose (ATA 2014), so the timing of the draw matters. Ask your clinician how to time your dose.
- Immunoassays and mass spectrometry can disagree on T3, most at the low end of the range (ATA 2014).
T3 has a much shorter half-life than T4 (about a day has been quoted for liothyronine, ATA 2014), so it can change within days of an illness, fasting or a change in medicine.
When to talk to a clinician
A result outside the range is not urgent by itself. See a clinician soon if you also have a fast or uneven heartbeat, shakiness, unplanned weight loss, heat intolerance or trouble sleeping. Call 911 or your local emergency number for chest pain, trouble breathing, confusion or fainting.
Questions you could ask
- Do I need total T3, or are TSH and free T4 enough for my situation?
- How does my total T3 compare with my TSH and free T4?
- Could pregnancy, birth control or my thyroid medicine explain this result?
- Do I need a free T3 test as well?
- Could a recent illness or low calorie intake explain a low result?
- Should we recheck this once I am feeling well?
Questions people ask
What is the difference between total T3 and free T3?
Total T3 counts all the T3 in your blood, including the part attached to carrier proteins. Free T3 counts only the small part that is free to act on your cells. Free T3 is less affected by things like pregnancy and birth control, but both are used.
Why can my T3 be low when I am ill or dieting?
Your body lowers T3 during serious illness and when you eat very little. This can happen even when the thyroid itself is working normally. Your clinician may recheck once you are well.
Is total T3 the same as T3 uptake?
No. They are two different tests. Total T3 measures the hormone. T3 uptake is an older test that gives an indirect picture of binding proteins and is rarely used today.
Show the science and sources
Units
- 1 ng/dL = 1 ng/dL
- 1 ng/dL = 0.01536 nmol/L
- 1 ng/dL = 0.01 ng/mL
Open the Total T3 unit converter
Is a change real?
Total T3 naturally varies about 9.6% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 31% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Biological variation estimates of thyroid related measurands - meta-analysis of BIVAC compliant studies (Fernandez-Calle P et al., EFLM WG-BV).
For clinicians and researchers
T3 is the main thyroid hormone that binds the nuclear receptor. T4 is a pro-hormone for T3, and the thyroid secretes T4 and T3 in a ratio of about 14 to 1, so most circulating T3 comes from conversion in tissues (ATA 2014). Less than 1% of T3 is unbound (Labcorp). Its half-life is much shorter than T4's (an average of about 1 day has been quoted for liothyronine, with individual studies reporting 6 to 22 hours), so doses of liothyronine or desiccated thyroid, which has a lower T4 to T3 ratio than the gland itself, produce peaks and troughs (ATA 2014). Total T3 is raised by binding-protein excess (pregnancy, estrogen, hereditary TBG increase) and lowered by nonthyroidal illness and fasting (Labcorp). Pregnancy raises TT3 and TT4 by about 50% through estrogen-driven TBG (Dong 2020). Labcorp runs total T3 by electrochemiluminescence immunoassay and cites Roche's reference intervals. LC-MS/MS and immunoassay share one marker here, but ATA 2014 notes they disagree most at the low end. Mayo's Thyroid Function Cascade (Monitor) reflexes to total T3 only when TSH is below 0.1 mIU/L and free T4 is in the usual range. Do not confuse total T3 with T3 uptake (Labcorp).
LOINC 3053-6 (primary), 14930-2. Specimen: serum-or-plasma.
Sources
- Triiodothyronine (T3) (test 002188), Labcorp
- Thyroid Function Cascade, Monitor (without Thyroid Peroxidase Antibody), Serum (THCSM), Mayo Clinic Laboratories (Extended Catalog)
- Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement, Thyroid (American Thyroid Association) (2014)
- The Need for Dynamic Clinical Guidelines: A Systematic Review of New Research Published After Release of the 2017 ATA Guidelines on Thyroid Disease During Pregnancy and the Postpartum, Frontiers in Endocrinology (2020)
- Liothyronine (PubChem compound 5920, C15H12I3NO4, 650.97 g/mol), PubChem, National Library of Medicine
- Thyroid function tests (MedlinePlus Medical Encyclopedia), MedlinePlus, National Library of Medicine
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.