Levothyroxine
hormoneSynthetic thyroid hormone (T4), the standard prescription treatment for an underactive thyroid.
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Lab test
What is a free T4 test, and what are normal free T4 levels?
A free T4 test measures the unbound, active form of thyroxine, the main hormone the thyroid makes. It is usually ordered with TSH, or after an abnormal TSH, to help assess thyroid function. ARUP Laboratories lists 0.9 to 1.7 ng/dL for adults aged 20 and older, with separate intervals in pregnancy.
What does the free T4 test measure?
T4 is the main hormone the thyroid makes. Free T4 is the active form that enters body tissues; bound T4 is attached to proteins and stays in the blood as a backup supply. MedlinePlus
A free T4 test is used more often than a total T4 test, and is usually done together with a TSH test. MedlinePlus
Why is the free T4 test ordered?
- After an abnormal TSH result. MedlinePlus
- To help diagnose hypothyroidism or hyperthyroidism, and to check levels in people taking thyroid hormone medicine. MedlinePlus
- To look into pituitary or hypothalamus conditions that change TSH. MedlinePlus
What are typical free T4 ranges?
ARUP Laboratories publishes free T4 intervals by age and trimester. ARUP Laboratories
Reference ranges differ between laboratories and testing methods, and many also differ by age, sex, pregnancy or the phase of the menstrual cycle. The range printed on your own report is the one that applies to your result, and a clinician interprets it alongside your history.
| Group | Reference interval |
|---|---|
| Adults, 20 years and older | 0.9 to 1.7 ng/dL |
| 12 to 19 years | 0.9 to 1.6 ng/dL |
| Pregnancy, 1st trimester | 0.9 to 1.4 ng/dL |
| Pregnancy, 2nd trimester | 0.7 to 1.3 ng/dL |
What can affect free T4 results?
- A T4 test alone cannot provide enough information to diagnose thyroid problems, which is why it is paired with TSH. MedlinePlus
- Biotin, often found in hair, skin and nail supplements, can interfere with some lab tests and cause incorrect results. Tell the person ordering the test if you take it. FDA
- Pregnancy changes the expected range. ARUP Laboratories
When should you talk to a clinician about a free T4 result?
- Results are read together with TSH and sometimes T3 or thyroid antibodies; discuss any result outside the printed range with the clinician who ordered it. MedlinePlus
DiscoverMedica explains what a test measures and what named sources publish. It does not interpret individual results; the clinician who ordered your test, or one you choose, does.
Where can you get a free T4 test?
A clinician can order it as part of routine or follow-up care. Some services also let you order lab tests yourself; check that a service’s panel includes this marker, which laboratory runs it, and who reviews the result with you.
- Function HealthAnnual membership for broad lab panels, drawn at Quest sites or at home.
- Quest HealthOrder individual lab tests from Quest Diagnostics online, without a doctor’s visit.
Free T4 Test: questions, answered
What is a normal free T4 level?
ARUP Laboratories lists 0.9 to 1.7 ng/dL for adults aged 20 and older, 0.9 to 1.4 ng/dL in the first trimester of pregnancy and 0.7 to 1.3 ng/dL in the second. Other labs and methods differ, so use the range printed on your report and discuss the result with the clinician who ordered it.
What is the difference between free T4 and total T4?
Total T4 measures thyroxine bound to proteins plus the free, unbound fraction. Free T4 measures only the active form that enters tissues. MedlinePlus notes that free T4 is considered more accurate and is used more often. Both are usually read together with TSH rather than on their own.
Why is free T4 ordered with TSH?
TSH and T4 move in opposite directions: when T4 is low, the pituitary makes more TSH, and when T4 is high it makes less. Looking at both helps a clinician tell a thyroid problem from a pituitary one, which a single result cannot do.
Sources
- MedlinePlus: Thyroxine (T4) Test
- ARUP Laboratories: Thyroxine, Free (Free T4) (test 0070138)
- FDA: Biotin Interference with Lab Tests
Last reviewed October 7, 2026. Every source above was checked on or before that date. Educational information only, not medical advice; it does not replace a conversation with a licensed clinician. DiscoverMedica does not employ clinicians.
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