Free Thyroxine (Free T4), Serum or Plasma, During Pregnancy
This test measures the amount of free thyroxine (free T4) in serum or plasma, the unbound thyroid hormone that is available to your body's tissues. In pregnancy, results are interpreted with pregnancy-specific reference ranges because normal hormone levels shift as pregnancy progresses.
["Free T4 testing during pregnancy helps assess thyroid function, but results must be interpreted using pregnancy-specific ranges and alongside TSH and your clinical context."]
Key facts at a glance
- LOINC
- 100978-6
- Short code
- Thyroxine.free.gestational SerPl IA-sCnc
- Synonyms
- Thyroxine.free.gestational SerPl IA-sCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 4.5
- Ref high
- 12
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- ia
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
What This Test Measures
Thyroxine, often called T4, is a hormone made by the thyroid gland. Most T4 in the blood is bound to carrier proteins, and only a small fraction circulates freely. Free T4 is the unbound portion that can enter tissues and influence metabolism, growth, and development. This test estimates that free portion in a serum or plasma sample.
Why It Is Ordered in Pregnancy
Pregnancy changes thyroid hormone binding and hormone production, so thyroid testing is often done when a person has symptoms, a known thyroid condition, or risk factors for thyroid disease. Free T4 is usually interpreted together with thyroid stimulating hormone (TSH), and sometimes thyroid antibodies, to evaluate whether the thyroid is working normally.
How the Sample Is Collected
A healthcare professional collects a blood sample, typically from a vein in your arm, into a tube that yields serum or plasma. You generally do not need special preparation unless your care team gives other instructions. Tell your provider about any thyroid medicines, supplements, or biotin products you take, since some can affect certain thyroid tests.
How Results Are Interpreted
Results are compared with reference ranges appropriate for the stage of pregnancy, because thyroid hormone patterns change across trimesters. A result outside the range does not by itself diagnose a thyroid disorder; clinicians consider TSH, symptoms, medical history, and sometimes antibody tests. If a result is unclear, your provider may repeat testing or order additional studies.
Reference range
NCBI: T4 4.5-12.0 µg/dLN/A
Take your thyroid medication after the blood draw if applicable. No fasting required.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
◆Associated findings1
Short name
Free T4, Pregnancy
Frequently asked questions
Do I need to fast before a free T4 blood test?
Can pregnancy change my free T4 results?
Does an abnormal free T4 result mean I have a thyroid problem?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy