Skip to content
Blood Test Tracker

Total T4 (Thyroxine)

Measures all the thyroid hormone thyroxine in your blood, both the part attached to carrier proteins and the small free part.

AI-verified against cited sources. Awaiting expert reviewAlso called TT4LOINC 3026-2

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Your thyroid makes thyroxine, or T4, as its main hormone. Almost all of it travels in your blood attached to carrier proteins, and a small part is free to act on your cells. Total T4 counts both parts together. Your body turns some T4 into T3, the more active form. Because total T4 includes the attached part, it changes when the amount of carrier protein changes, not only when the thyroid changes.

Why it matters

Total T4 is one of several tests that show how much hormone your thyroid is releasing. It is an older, widely available test, and many clinicians now start with TSH and free T4 instead, because free T4 is less affected by carrier proteins. Total T4 still appears on the classic thyroid panel, usually next to T3 uptake and a free thyroxine index. It is easiest to understand next to your TSH and free T4.

If it’s low

A lower total T4 is linked to an underactive thyroid. It can also be low when the thyroid is fine but your carrier proteins are low, or during a serious illness.

Common causes

  • An underactive thyroid, such as Hashimoto's thyroiditis
  • Serious or long-term illness, which can lower hormone levels without a thyroid problem
  • Low carrier protein, for example from a kidney condition that loses protein in the urine, from liver disease, or from an inherited TBG shortage
  • Androgens, anabolic steroids or prednisone
  • Some medicines, including phenytoin, salicylates and carbamazepine
  • Too little signal from the pituitary gland to the thyroid (a rarer cause, called central hypothyroidism)

What can help

  • Ask your clinician to read it together with TSH and free T4
  • Tell your clinician about recent illness, new medicines or steroid use
  • If you take thyroid medicine, ask whether a recheck of your levels is due

If it’s high

A higher total T4 is linked to an overactive thyroid. It can also be high when the thyroid is working normally but your blood carries more binding protein than usual. This is common in pregnancy and with estrogen.

Common causes

  • Pregnancy: estrogen raises carrier protein, and total T4 rises by about 50%
  • Estrogen-containing birth control or hormone therapy
  • Thyroid hormone medicine: a blood draw soon after a dose, or a dose that is more than your body needs
  • Inherited changes in carrier proteins, such as extra TBG or an albumin variant, that raise total T4 without a thyroid problem
  • Liver conditions such as hepatitis, which can raise carrier protein
  • High iodine intake, or certain medicines such as amiodarone
  • An overactive thyroid, such as Graves' disease, or the early stage of thyroid inflammation

What can help

  • Ask how your total T4 looks next to your TSH and free T4
  • Tell your clinician if you are pregnant, take birth control or hormone therapy, or take thyroid medicine
  • Ask whether a free T4 test would give a clearer picture
  • Mention any biotin, iodine or other supplements you take

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.

The usual range depends on age. Choose an age to see the one that applies.

Children and teens 3 ranges

See Total T4 by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Pregnancy: total T4 rises by about 50% because estrogen raises carrier proteins
  • Estrogen from birth control or hormone therapy raises carrier proteins, while androgens and anabolic steroids lower them
  • Kidney, liver or other serious illness can change binding proteins and hormone levels
  • Medicines such as amiodarone, high-dose propranolol, salicylates, phenytoin and carbamazepine
  • Age: lab ranges differ for children and teens
  • Timing of levothyroxine: T4 can run about 15% higher around 4 hours after a dose
  • Biotin supplements, on some immunoassays
  • 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.
  • Anything that changes the amount of carrier protein (TBG) moves total T4 without a change in the thyroid: pregnancy, estrogen-containing birth control or hormone therapy, androgens and anabolic steroids, kidney or liver conditions, and inherited TBG differences (Labcorp).
  • Some medicines change total T4, including amiodarone and high-dose propranolol (higher) and salicylates, phenytoin and carbamazepine (lower) (Labcorp).
  • If you take levothyroxine, T4 can peak about 15% above its usual level around 4 hours after a dose (ATA 2014). Ask your clinician how to time your dose around the blood draw.

About 6 weeks to settle at a new steady state after a change in thyroid medicine, because T4 has a half-life of about 7 days (ATA 2014).

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 or heat intolerance, or marked tiredness, feeling cold, constipation or swelling at the front of the neck. Call 911 or your local emergency number for chest pain, trouble breathing, confusion or fainting.

Questions you could ask

  • Do I need total T4, or are TSH and free T4 enough for my situation?
  • How does my total T4 compare with my TSH and free T4?
  • Could pregnancy, birth control or hormone therapy explain this result?
  • Would a free T4 test give a clearer answer?
  • How does this result look next to my TSH and free T4?
  • Could an illness or a medicine be changing my binding proteins?

Questions people ask

What is the difference between total T4 and free T4?

Total T4 counts all the thyroxine in your blood, including the part attached to carrier proteins. Free T4 counts only the small part that is not attached and can act on your cells. Free T4 is less affected by things like pregnancy and birth control, so many clinicians use it more often.

Can my total T4 be high or low even if my thyroid is fine?

Yes. Pregnancy, estrogen-containing medicines, some inherited traits, kidney or liver conditions and some medicines change the amount of carrier protein. That moves total T4 up or down without a change in how your thyroid is working. A free T4 and TSH test usually helps sort this out.

Should I stop biotin before this test?

High-dose biotin can interfere with some thyroid tests. Labcorp asks patients to stop biotin at least 72 hours before the draw. Check with your clinician or lab before changing any supplement.

Show the science and sources

Units

  • 1 µg/dL = 1 µg/dL
  • 1 µg/dL = 12.87 nmol/L
  • 1 µg/dL = 10 ng/mL

Open the Total T4 unit converter

Is a change real?

Total T4 naturally varies about 6.4% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 24% 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

Total T4 (TT4) is protein-bound plus free thyroxine. Nearly all circulating T4 is bound, mainly to TBG, transthyretin and albumin, so TT4 tracks binding-protein concentration as well as thyroid output. This is why TSH, with free T4 when needed, is preferred first-line (ETA 2018), and why ARUP calls free T4 the preferred alternative to T uptake and the free thyroxine index. T4 is a pro-hormone for T3 and is secreted at about 14 times the amount of T3. Its half-life is about 7 days, so steady state after a dose change takes about 6 weeks (ATA 2014). Labcorp runs TT4 by electrochemiluminescence immunoassay. Binding-protein changes that raise TT4 include pregnancy, estrogens, hereditary TBG excess, hepatitis and familial dysalbuminemic hyperthyroxinemia. Androgens, anabolic steroids, prednisone, nephrotic syndrome and TBG deficiency lower it (Labcorp). Estrogen in pregnancy raises TBG and TT4 by about 50% (Dong 2020). In NHANES III the mean TT4 in people without thyroid disease was 112.3 nmol/L (about 8.7 µg/dL), and was higher in Mexican Americans (Hollowell 2002).

LOINC 3026-2 (primary), 14921-1, 35226-0. Specimen: serum-or-plasma.

Sources

  1. Thyroxine (T4) (test 001149), Labcorp
  2. Thyroxine, Total T4 (test 3005978), ARUP Laboratories
  3. Free Thyroxine Index Panel (test 3005976), ARUP Laboratories
  4. Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): NHANES III, Journal of Clinical Endocrinology & Metabolism (2002)
  5. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement, Thyroid (American Thyroid Association) (2014)
  6. 2018 European Thyroid Association Guideline for the Management of Graves' Hyperthyroidism, European Thyroid Journal (European Thyroid Association) (2018)
  7. 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)
  8. Thyroxine (PubChem compound 5819 / levothyroxine, C15H11I4NO4, 776.87 g/mol), PubChem, National Library of Medicine
  9. Thyroid function tests (MedlinePlus Medical Encyclopedia), MedlinePlus, National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

Calculators

Related markers