| A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # |
Free T4
Test CodeFT4
Alias/See Also
Free T4 Free Thyroxine
T4 Uptake
T4 Uptake
CPT Codes
84439
Preferred Specimen
1.0 mL serum or plasma.
Serum is preferred if the sample is used as part of a TSH Cascade
Serum is preferred if the sample is used as part of a TSH Cascade
Minimum Volume
0.5 mL
Other Acceptable Specimens
Plasma - Li Heparin (PST)
Plasma –EDTA (Lavender)
Gold top (SST)
Tiger top (SST)
Red top
Plasma –EDTA (Lavender)
Gold top (SST)
Tiger top (SST)
Red top
Transport Temperature
Room temperature
Specimen Stability
Room temperature: 5 days at 20-24ºC
Refrigerated: 7 days at 2-8°C
Frozen: 1 month at 20°C, Freeze only once.
Refrigerated: 7 days at 2-8°C
Frozen: 1 month at 20°C, Freeze only once.
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Samples not labeled with complete first and last name and date of birth.
Grossly hemolyzed.
Grossly hemolyzed.
Methodology
Electrochemiluminescence immunoassay.
Setup Schedule
Sun-Sat
Report Available
Same day
Reference Range
0.93 - 1.70 ng/dL
Clinical Significance
Thyroxine (T4) is the main thyroid hormone secreted into the bloodstream by the thyroid gland. Together with triiodothyronine (T3) it plays a vital role in regulating the body’s metabolic rate, influences the cardiovascular system, growth and bone metabolism, and is important for normal development of gonadal functions and nervous system. T4 circulates in the bloodstream as an equilibrium mixture of free and serum bound hormone. Free T4 (fT4) is the unbound and biologically active form, which represents only 0.03 % of the total T4. The remaining T4 is inactive and bound to serum proteins such as thyroxine binding globulin (75 %), pre-albumin (15 %), and albumin (10 %).
The determination of free T4 has the advantage of being independent of changes in the concentrations and binding properties of the binding proteins; additional determination of a binding parameter (T-uptake, TBG) is therefore unnecessary. Therefore free T4 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status. It should be measured together with TSH if thyroid disorders are suspected and is also suitable for monitoring thyrosuppressive therapy.
The determination of free T4 has the advantage of being independent of changes in the concentrations and binding properties of the binding proteins; additional determination of a binding parameter (T-uptake, TBG) is therefore unnecessary. Therefore free T4 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status. It should be measured together with TSH if thyroid disorders are suspected and is also suitable for monitoring thyrosuppressive therapy.
Performing Laboratory
Frederick Health Laboratory
400 W 7th Street
Frederick, MD 21701
Last Updated: September 27, 2023
