Free Thyroxine

Label Mnemonic:	FT4
Epic code:	LAB127
Order form:	Laboratory Requisition
Supply order:	Supply Order Form
Billing:	Billing Policies
CPT code:	84439

Specimen(s):

Plasma

Collection Medium:

Plasma Separator Tube 4.5 mL

Alternate Collection Media:

Call laboratory for additional acceptable specimen collection containers.

Minimum:

3 mL whole blood in light green top tube or TWO Microtainer^® devices

Testing Schedule:

24 hrs/day, 7 days a week, including holidays.

Turn Around Time:

1 hour (upon receipt in laboratory)

Reference Range:

Age	Range
0 up to 1 month	1.2 – 3.9 ng/dL
1 – <12 months	1.1 – 1.7 ng/dL
1 – <19 years	1.0 – 1.6 ng/dL
19+ years	0.8 – 1.8 ng/dL

Ranges approved by Laboratory Director. Pediatric reference intervals are from the CALIPER study

Bohn MK, Higgins V, Asgari S, Leung F, Hoffman B, Macri J, Adeli K. Paediatric reference intervals for 17 Roche cobas 8000 e602 immunoassays in the CALIPER cohort of healthy children and adolescents. Clin Chem Lab Med. 2019 Nov 26;57(12):1968-1979. doi: 10.1515/cclm-2019-0707. PMID: 31377737.

Comments:

Please print, complete and submit the Advance Beneficiary Notice (ABN) along with the Laboratory Requisition before shipping the specimen.

Pediatric Reference ranges updated 4/6/2026.

Test Limitations:

The assay is unaffected by icterus (bilirubin is less than 41 mg/dL), hemolysis (Hb is less than 1000 mg/dL), lipemia (Intralipid is less than 2000 mg/dL) and biotin is less than 1200 ng/mL (criterion: recovery within plus or minus 10% of initial value).

No interference was observed from rheumatoid factor (up to 1200 U/mL) and samples from dialysis patients.

No significant interference was demonstrated by high concentrations of 17 commonly used pharmaceuticals tested in vitro. At typical therapeutic concentrations, furosemide, carbamazepine, phenytoin, and levothyroxine sodium (L-T4, synthetic levothyroxine) caused elevated free T4 recovery.

The test cannot be used to determine fT4 in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should be interrupted for 4-6 weeks to allow the physiological state to become re-established.*

Samples from neonates have not been tested with the FT4 assay.

Auto-antibodies to thyroid hormones can interfere with the assay.

The risk of interference from potential immunological interactions between test components and rare sera has been minimized by the inclusion of additives. In rare cases, interference due to extremely high titers of antibodies to streptavidin ruthenium can occur.

For diagnostic purposes, the FT4 findings should always be assessed in conjunction with the patient's medical history, clinical examination and other findings.

*Bantle JP, Hunninghake DB, Frantz ID, Kuba K, Mariash CN, Oppenheimer JH. Comparison of Effectiveness of Thyrotropin-Suppressive Doses of D- and L-Thyroxine in Treatment of Hypercholesterolemia. Am J Med 1984;3:475-481.

Methodology:

Electrochemiluminescence Immunoassay

Assay updated with increased tolerance to biotin on 4-16-2024.

Sample Processing:

Centrifuge at a speed and time necessary to get barrier separation of plasma/serum and cells within 1 hour of collection. Send specimen in original tube. Do Not transfer to another tube.
Each sample must be labeled with at least TWO full patient identifiers (First/Last Name & DOB are sufficient for non-UIHC affiliated clients) to avoid sample rejection/delays.

Sample Storage:

Refrigerate.
All sample storage requirements are intended for delivery to UIHC within 24 hours of collection for testing. If samples won't arrive in this time period, please call the UIHC Core Lab for alternative storage/shipping instructions (319-356-3527).

Transport Instructions:

Place labeled specimen into zip-lock type biohazard bag; seal bag.
Place completed requisition into outside pocket of bag.
Transport in cooler with refrigerated coolant packs.