Free Thyroxine (FT4)

CPT Code:  



Serum or heparinized plasma 


0.8 mL 

Minimum Volume: 

0.3 mL (Note: This volume does not allow for repeat testing.) 


Red-top tube or gel-barrier tube




If a red-top tube is used, transfer separated serum to a plastic transport tube.

Specimen should be free of particulate matter. To prevent the appearance of fibrin in serum samples, complete clot formation should take place before centrifugation. If clotting time is increased due to thrombolytic or anticoagulant therapy, the use of plasma specimen will allow for faster sample processing and reduce the risk of particulate matter.

Blood collected in the presence of oxalate potentially can cause clumping of chrome particles and should not be used.

Frozen plasma samples with insufficient anticoagulants potentially can cause clumping of chrome particles and should not be used.

Reference Range:



Temperatures  Period 
Room Temperature  24 Hours 
Refrigerated (2-8 ⁰C)  14 Days 
Frozen (-20 ⁰C)  30 Days 
Avoid repeated freezing and thawing.


Use: Thyroxine (T4) is the primary hormone secreted by the thyroid gland. It regulates the body’s rate of metabolism and is necessary for normal growth and development.
Clinical Significance: High values are usually due to excessive T4 production, called hyperthyroidism.

Low values usually indicate too little T4 is being produced by the thyroid gland, called hypothyroidism. Treatment is often thyroid medication.

Causes for Rejection: Citrate plasma specimen; improper labeling


Interfering Substances The following substances can cause elevated FT4 results by releasing T4 from serum binding protein.  
Substance  Test Concentration  SI Units 
Carbamazepine  12 mg/dL  508 µmol/L 
Furosemide 2 mg/dL  61 µmol/L 
Ibuprophen 10 mg/dL 484 µmol/L
Phenytoin 10 mg/dL 396 µmol/L
Salicylic acid 50 µg/mL 3.62 mmol/L
Valproic acid 50 mg/dL 3467 µmol/L

Patient samples may contain heterophilic antibodies that could react in immunoassays to give falsely elevated or depressed results. This assay has been designated to minimize interference from heterophilic antibodies.