Thyroxine, Free (Free T4)

Casandra

Casandra Test Code AR47562Version 1 (DRAFT)

Performing Lab

Lab ARUP LaboratoriesLab Test ID 0070138CPT Code 84439

Thyroxine, Free (Free T4)

Also known as: FT4

Clinical Use

Order Test

Use

This test measures the level of free thyroxine (Free T4) in the blood, which is important for evaluating thyroid function and diagnosing thyroid disorders. It is particularly useful following an abnormal thyroid stimulating hormone (TSH) result to diagnose thyroid disease and in cases of suspected secondary hypothyroidism, where it is ordered in conjunction with TSH. Free T4 is the unbound form of thyroxine and provides a more accurate indication of thyroid hormone status than total T4, which includes both bound and unbound hormone.

Special Instructions

This test is not preferred for initial thyroid disorder screening. For initial screening, refer to the Thyroid Stimulating Hormone test (0070145). Ensure proper specimen collection and handling to maintain sample integrity. Separate serum or plasma from cells as soon as possible after collection, ideally within two hours, and store the specimen refrigerated.

Limitations

The test may not be suitable for patients with altered binding protein levels, as this can affect the measurement of free thyroxine. Grossly hemolyzed specimens are unacceptable and should be rejected. It is important to maintain proper storage temperatures and timelines to prevent degradation of the sample, which may affect test results. Free T4 levels can be affected by a variety of factors including illness, medications, and pregnancy, which should be considered when interpreting results.

Test Details

FDA Approved