Free Thyroxine (Free T4)

Additional Information:

EPIC Test Name: T4 FREE

CPT Code(s):

84439

Collection Procedure:

Collect specimen in SST tube. Allow to clot. Centrifuge and store refrigerated. Transport to lab.

Specimen Requirements:

Type: Serum (preferred) or Plasma
Container/Tube: SST or Lithium Heparin (Green top)
Sample Volume: 1 mL
Minimum Volume: 0.2 mL (Repeat testing is not possible with this specimen volume.)
Storage:Refrigerate
Stability (collection to time of analysis/testing):

  • Refrigerated: 2 days
  • Frozen: 2 months

Use:

The principal thyroid hormone, thyroxine (T4), circulates almost entirely bound to carrier proteins, chief of which is thyroxine-binding globulin (TBG). Altered carrier protein concentrations induce changes in total T4 levels, and free T4 concentrations tend to stay within a tight range. For this reason, total T4 measurements do not always reflect thyroid status. TBG levels may vary under different physiological conditions, such as during pregnancy, oral contraceptive use, and estrogen therapy. Total T4 levels may increase above the normal range while free T4 remains normal. Alternatively, patients with a dysfunctional thyroid gland and altered TBG levels can have normal total T4 levels, masking the illness. Since abnormal T4 levels may signify either abnormal thyroid function or carrier protein variation (physiological or pathological), free T4 measurements morehighly correlate with thyroid status than total T4 measurements.

Methodology:

Solid-phase, enzyme-labeled chemiluminescent competative immunoassay

Reported:

1-3 Business Days

Reference Values:

0.89-1.76 ng/dL

Performed:

Monday - Friday