CPT:84439
Free T4 may be indicated when binding globulin (TBG) problems are perceived, or when conventional test results seem inconsistent with clinical observations. It is normal in subjects with high thyroxine-binding globulin hormone binding who are euthyroid (ie, free thyroxine should be normal in nonthyroidal diseases). It should be normal in familial dysalbuminemic hyperthyroxinemia.
FT4 may be increased with radiologic contrast agents, propranolol, amiodarone, and heparin. It may be decreased with carbamazepine (Tegretol®). Free T4 is a small part of total T4. Increased free T4 levels may occur in subjects with nonthyroid diseases. Such elevations are described as transient. Low values were reported in patients with nonthyroidal illness.3 Discrepancies in free T4 levels between methods are recognized. Reliability problems continue to be discussed with the direct (analog) methods. Results of kits intended to serve in place of equilibrium dialysis technique may differ from the reference method.
Competitive immunoassay
Serum
0.8 ML
0.3 mL (Note: This volume does not allow for repeat testing.)
SST (Serum-separating tube)
Heparin has been reported to have in vivo and in vitro effects on free T4 assay. Hence samples should not be collected during or soon after the administration of this anticoagulant.
If a red-top tube is used, transfer separated serum to a plastic transport tube.
Maintain specimen refrigerated
| Temperature | Period |
|---|---|
| Room temperature | unspecified |
| Refrigerated | ≤ 7 days |
| Frozen | ≤ 4 weeks |
| Freeze/thaw cycles | Avoid |
Range: 0.78 – 2.44 ng/dL
Improper labeling
This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.
