Free Thyroxine

Label Mnemonic:	FT4
Epic code:	LAB127
Downtime form:	Doctor/Provider Orders - Pathology Core and Specialty Care Nursery

Chemistry
6240 RCP
356-3527

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:

Reference range for 5 years to adult changed on 11/26/2013.
Reference range for 18 years and older changed on 2/23/2016.

Pediatric Reference ranges updated 4/6/2026.

This test is also performed at the Iowa River Landing (IRL) and North Dodge clinical laboratories (for specimens drawn at those sites).

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.

CPT Code:

84439