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T4, Free
MessagePerforming Lab: Central Lab, Hudson, Lakeview, Regions, Sartell, Westfields
Test Code
3100
Alias/See Also
Sunquest: T4FRE; Thyroxine Free, Free T4
CPT Codes
84439
Preferred Specimen
0.3 mL Lithium Heparin Plasma (light green-top) tube
Minimum Volume
0.2 mL
Other Acceptable Specimens
Serum (gold-top) tube
Transport Temperature
Refrigerated
Specimen Stability
Room temperature (24 hours), Refrigerated (6 days), Frozen (6 days)
Methodology
Chemiluminescent microparticle immunoassay (CMIA)
Setup Schedule
Hospital: Daily
Clinics: Monday - Friday
Clinics: Monday - Friday
Report Available
Same Day
Limitations
Reference Range
0.7 - 1.5 ng/dL
Clinical Significance
The free fraction of the circulating thyroxine (T4) is considered to exert the main influence on metabolic control. Consequently, the FT4 concentration is believed to be the most direct indicator of an individual’s thyroid status. FT4 concentrations are generally depressed in hypothyroidism and raised in hyperthyroidism. Measurement of FT4 thus provides an aid to the differential diagnosis of thyroid disease.
FT4 concentrations are independent of the concentration of thyroid hormone binding proteins and may therefore be measured in patients with elevated or reduced binding protein concentrations without the need for additional tests of binding capacity. In borderline cases of suspected thyroid malfunction, additional tests such as free T3 or TSH may be necessary.
FT4 concentrations are independent of the concentration of thyroid hormone binding proteins and may therefore be measured in patients with elevated or reduced binding protein concentrations without the need for additional tests of binding capacity. In borderline cases of suspected thyroid malfunction, additional tests such as free T3 or TSH may be necessary.
