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HelpTest Code T4 T4 (THYROXINE)
Methodology
Competitive Chemiluminometric Immunoassay
Elevated T4 levels are seen in hyperthyroidism and acute thyroiditis. Low T4 levels are seen in hypothyroidism, myxedema, cretinism, chronic thyroiditis, and occasionally, subacute thyroiditis.
T4 levels are useful for monitoring treatment with synthetic hormones (synthetic T3 will cause a low total T4), and for monitoring treatment of hyperthyroidism with thiouracil and other antithyroid drugs. Total T4 levels offer a good index of thyroid function when the thyroxine-binding globulin (TBG) is normal and nonthyroidal illness is not present.
Note: If a free thyroxine index (FTI) is desired order T7 “T7, Plasma or Serum.”
Specimen Requirements
Submit only 1 of the following specimens:
Preferred:
Serum
Serum Gel Tube
Container/Tube: Serum gel tube(s)
Specimen Volume: 1 mL (minimum volume: 0.5 mL) of
serum
Collection Instructions: Spin down within 4 hours of draw.
Note: Label specimen appropriately (serum).
Alternate:
Serum
Plain, Red-Top Tube
Container/Tube: Plain, red-top tube(s)
Specimen Volume: 1 mL (minimum volume: 0.5 mL) of
serum
Collection Instructions: Spin down within 4 hours of draw.
Note: Label specimen appropriately (serum).
Alternate:
Plasma
Container/Tube: Green-top (lithium heparin) tube(s)
Specimen Volume: 1 mL (minimum volume: 0.5 mL) of plasma
Collection Instructions: Spin down within 4 hours of draw.
Note: Label specimen appropriately (plasma).
Day(s) Test Set Up
Monday through Friday
Reference Values
≤ 3 Days 8.0 – 18.0 μg/dL
3 – 30 Days 5.0 – 15.0 μg/dL
1 Month – < 1 Year 6.0 – 14.0 μg/dL
≥ 1 Year – 4 Years 4.5 – 11.0 μg/dL
≥ 5 Years – 17 Years 4.5 – 10.0 μg/dL
≥ 18 Years 5.5 – 11.0 μg/dL
Performing Laboratory
Logan Health Medical Center-Chemistry
Test Classification and CPT Coding
84436
Specimen Transport Temperature
Refrigerate