Total T4 blood test
Total thyroxine
In short
A typical adult total T4 result is 4.5 to 11.7 µg/dL. A high total T4 can come from extra carrier protein in the blood rather than an overactive thyroid. Free T4 and TSH help tell the two apart. Always read your result against the range printed on your own report.
All the thyroxine in the blood, bound and free together.
Total T4 counts all the thyroxine, the thyroid's main hormone, including the part carried on proteins. Because it follows those carrier proteins, pregnancy, birth control pills and some illnesses can shift it. Many doctors now prefer free T4, which these changes affect less.
Is my Total T4 high or low?
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Typical range
| For | Range | Unit |
|---|---|---|
| Adults | 4.5–11.7 | µg/dL |
| Adults | 58–151 | nmol/L |
Typical ranges for adults. Labs differ a little, so the range printed next to your result is the one to go by.
If your Total T4 is high
A high total T4 can come from extra carrier protein in the blood rather than an overactive thyroid. Free T4 and TSH help tell the two apart.
Common reasons
- Birth control pills or hormone therapy with estrogen, which raise carrier proteins
- Pregnancy, which raises the proteins that carry T4
- For people on thyroid medicine, a dose above what the body needs
- An overactive thyroid, for example from Graves' disease, an immune condition
- Thyroiditis, an inflamed thyroid that leaks stored hormone for a few weeks
- Less often: an inherited difference in the proteins that carry T4
If your Total T4 is low
A low total T4 can come from low carrier protein rather than a slow thyroid. It can also go with an underactive thyroid, when TSH is usually high.
Common reasons
- Low carrier protein, for example with some kidney conditions
- Some medicines, including steroid medicines and some seizure medicines
- A recent serious illness, which can lower thyroid tests for a while
- Hashimoto's, a common immune condition that gradually slows the thyroid
- Less often: a problem in the pituitary, the gland that makes TSH
What can throw it off
Pregnancy, birth control pills and hormone therapy raise total T4, and biotin, a hair-and-nail supplement, can distort it on some lab tests.
Questions to take to your appointment
- 1Would a free T4 test give a clearer answer?
- 2Could birth control pills or other hormones explain this?
- 3What does my TSH show alongside it?
Read together with
Doctors rarely read Total T4 alone. These usually sit next to it.
Quick answers
What is the total T4 blood test?
Total T4 (Total thyroxine): All the thyroxine in the blood, bound and free together. Total T4 counts all the thyroxine, the thyroid's main hormone, including the part carried on proteins. Because it follows those carrier proteins, pregnancy, birth control pills and some illnesses can shift it. Many doctors now prefer free T4, which these changes affect less.
What is a normal total T4 level?
For most adults, total T4 is typically 4.5 to 11.7 µg/dL. Some countries print it in nmol/L, so check the unit first. Labs differ a little, so the range printed next to your result is the one to go by.
What does a high total T4 mean?
A high total T4 can come from extra carrier protein in the blood rather than an overactive thyroid. Free T4 and TSH help tell the two apart. Common reasons include birth control pills or hormone therapy with estrogen, which raise carrier proteins; pregnancy, which raises the proteins that carry T4; for people on thyroid medicine, a dose above what the body needs; and an overactive thyroid, for example from Graves' disease, an immune condition. A doctor reads it together with your other results and how you feel.
What does a low total T4 mean?
A low total T4 can come from low carrier protein rather than a slow thyroid. It can also go with an underactive thyroid, when TSH is usually high. Common reasons include low carrier protein, for example with some kidney conditions; some medicines, including steroid medicines and some seizure medicines; a recent serious illness, which can lower thyroid tests for a while; and Hashimoto's, a common immune condition that gradually slows the thyroid.
What can affect my total T4 result?
Pregnancy, birth control pills and hormone therapy raise total T4, and biotin, a hair-and-nail supplement, can distort it on some lab tests.
What should I ask my doctor about my total T4?
Questions worth taking to the appointment: Would a free T4 test give a clearer answer? Could birth control pills or other hormones explain this? What does my TSH show alongside it?
What this page can’t do
It reads each number on its own.
Doctors read results together — ALT with AST, ferritin with hemoglobin, TSH with free T4. A pattern says more than any single flag.
It doesn't know you.
Your age, your medicines, how you feel and your last result all change what a number means.
It doesn't remember.
Close this page and your results are gone. That's on purpose: nothing was ever sent to us.
We also make Vita: Health Coach, an app for those parts. It reads your whole report with your medicines and history in mind, answers follow-up questions in plain words, and keeps your reports in one place — for you and for the people you look after.
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