T3 Uptake
Shows how many binding spots on your blood proteins are open for thyroid hormone. It is not a hormone level, and it is used to work out the free thyroxine index.
AI-drafted. Awaiting verificationAlso called T3U, T3RU, THBRLOINC 3050-2
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What it is
T3 uptake does not measure T3 in your blood. It estimates how many of the binding spots on your thyroid hormone carrier proteins are still open. Most thyroid hormone in blood is attached to carrier proteins, mainly one called thyroxine-binding globulin (TBG). The lab adds a small amount of labeled hormone and sees how much is taken up. On the traditional percent scale, more open spots gives a lower number, and fewer open spots gives a higher one. Labs use the result together with total T4 to calculate the free thyroxine index.
Why it matters
Carrier proteins change with pregnancy, hormone medicines and some illnesses, and that can make total T4 look high or low. T3 uptake helps adjust for this. It is read together with total T4 and TSH, not on its own. Many labs now measure free T4 directly, so T3 uptake is less common than it used to be.
If it’s low
On the traditional percent scale, a result below your lab's range means more binding spots are open. That can happen when the thyroid is underactive, because there is less hormone to fill the spots. It can also happen when carrier proteins are high, as in pregnancy or with estrogen-containing medicines. A low result in pregnancy is expected.
Common causes
- Pregnancy
- Estrogen-containing medicines, such as some birth control pills and menopausal hormone therapy
- An underactive thyroid
- A few other medicines that raise carrier proteins, such as methadone
- Inherited high levels of thyroxine-binding globulin (rare)
What can help
- Ask how this number fits with your total T4, free thyroxine index and TSH
- Tell your clinician if you are pregnant or take hormone medicines
If it’s high
On the traditional percent scale, a result above your lab's range means fewer binding spots are open. That can happen when the thyroid is overactive, because extra hormone fills the spots. It can also happen when carrier proteins are low, for example with androgen or glucocorticoid medicines. On its own it does not say which, so it is read with total T4 and TSH.
Common causes
- An overactive thyroid
- Lower levels of carrier proteins, including from androgen or glucocorticoid medicines
- Inherited low levels of thyroxine-binding globulin (rare)
- Some long-term illnesses that lower blood proteins
- Biotin supplements, which can change some test versions
What can help
- Ask how this number fits with your total T4, free thyroxine index and TSH
- Tell your clinician about every medicine and supplement you take, including hormone medicines and biotin
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.
For adults: 22–35 %. Source: Cureus, Thyroid (American Thyroid Association), Case Reports in Endocrinology.
- Adults (this one applies)22–35 %
Sources: Cureus, Thyroid (American Thyroid Association), Case Reports in Endocrinology
What can affect your result
- Pregnancy and estrogen-containing medicines, which raise carrier proteins
- Androgens and glucocorticoids, which lower carrier proteins
- An overactive or underactive thyroid
- How the lab reports it: percent or ratio, since the scales differ
- Biotin supplements
- Test method and lab
- Pregnancy and medicines that raise thyroid-binding proteins (estrogen, clofibrate, opiates such as methadone, 5-fluorouracil, perphenazine) lower traditional T3 uptake.
- Androgens and glucocorticoids, which lower binding proteins, raise it.
- Medicines that pull thyroid hormone off its carrier proteins (salicylates, phenytoin, carbamazepine, furosemide, NSAIDs) can change the result, and heparin can do the same in the test tube.
- High-dose biotin can interfere with some immunoassay versions of the test.
Follows changes in binding proteins and thyroid hormone, so it can take weeks to settle after a change in medicine or after pregnancy ends.
When to talk to a clinician
T3 uptake is not meant to be read alone, and no single cutoff marks an urgent result. Talk with a clinician about any result outside your lab's range, especially if you have symptoms of an overactive or underactive thyroid. If you have chest pain, a very fast or irregular heartbeat, fainting or confusion, call 911 or your local emergency number.
Questions you could ask
- Do I need a T3 uptake test, or is a direct free T4 test enough?
- Does this fit with my total T4, free thyroxine index and TSH?
- Could a medicine I take change my carrier proteins?
- Could pregnancy or a hormone medicine explain this?
- Does this fit with my total T4, free thyroxine index and TSH?
Questions people ask
Is T3 uptake the same as my T3 level?
No. It does not measure the T3 hormone. It reflects how many binding spots on carrier proteins are open, and it is used with total T4 to calculate the free thyroxine index. Total T3 and free T3 are separate tests.
Why is my T3 uptake low in pregnancy?
Pregnancy raises the carrier proteins that bind thyroid hormone. More binding spots are open, so the traditional T3 uptake result goes down. This is expected. Your clinician can look at it together with your other thyroid tests.
Show the science and sources
For clinicians and researchers
T3 resin uptake is an indirect index of unoccupied binding sites, mainly on TBG. On the traditional scale, high TBG leaves more open sites and lowers uptake, while low TBG or extra hormone raises it. Used with total T4 it gives the free thyroxine index (T4 × T3 uptake % ÷ 100). The ATA-commissioned 2023 review says the original uptake tests were developed to adjust total T4 for binding protein changes, and that current tests, renamed thyroid hormone binding ratios, mostly use automated immunological formats and compare available binding sites with a reference assigned a value of 1.00 or 40%, depending on method. Scales are therefore method-specific. One hospital using a Roche Elecsys analyzer reports a T-uptake ratio with a 0.7 to 1.2 interval and divides total T4 by it to get the index, so on that analyzer the number moves with binding capacity, not against it as the traditional percent does. Do not convert between a ratio and a percent without the lab's own reference. Table 2 of the review lists drugs that raise binding proteins (estrogen, clofibrate, opiates, 5-fluorouracil, perphenazine) and drugs that lower them (androgens, glucocorticoids).
LOINC 3050-2 (primary), 74794-9. Specimen: serum-or-plasma.
Sources
- Thyroid Stimulating Hormone and Thyroid Hormones (Triiodothyronine and Thyroxine): An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status, Thyroid (American Thyroid Association) (2023)
- Post-COVID Myoclonus-Ataxia Syndrome: A Case Report of Successful Recovery With Intravenous Immunoglobulin (IVIG) Treatment (Table 3: laboratory thyroid panel with reference ranges), Cureus (2025)
- A Rare Case of Primary Thyroid Lymphoma (Table 1: laboratory results with reference ranges), Cureus (2026)
- Measurement of Serum Free Thyroxine Index May Provide Additional Case Detection Compared to Free Thyroxine in the Diagnosis of Central Hypothyroidism, Case Reports in Endocrinology (2015)
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.