T4, Free CPT: 84439

Special Instructions

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.

Expected Turnaround Time

Within 1 day

Specimen Requirements




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.

Storage Instructions

Room temperature

Stability Requirements

Temperature Period
Room Temperature Not recommended
Refrigerated 7 days
Frozen 4 weeks
Freeze/thaw/ cycles Stable x 3

Patient Information

Patient Preparation

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.

To help evaluate thyroid gland function; to help diagnose thyroid disease; to monitor effectiveness of thyroid treatment; sometimes a free T4 is used to help diagnose congenital hypothyroidism in newborns.

When you have signs and symptoms of thyroid disease and/or an enlarged thyroid (goiter) or when you have a thyroid nodule (a small lump on the thyroid gland that may be solid or fluid-filled cyst), usually after an abnormal result on a TSH test; when you are being treated for a thyroid disorder.

A blood sample drawn from a vein in your arm or from pricking the heel of an infant.

None needed; however, certain medications, multivitamins and supplements can interfere with the free T4 test, so tell your healthcare provider about any prescribed or over-the-counter drugs and/or supplements that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day. Acute illness may affect thyroid testing results. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness.