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| 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 # |
THYROXINE, FREE
Test CodeLAB127
Alias/See Also
FREE T4
CPT Codes
84439
Preferred Specimen
Green Top/Gel Li Hep
Minimum Volume
0.5 mL
Other Acceptable Specimens
7 mL Red Top Tube
Pediatric Red
Pediatric Green
Pediatric SST
Pediatric Red
Pediatric Green
Pediatric SST
Transport Temperature
Refrigerate specimen
Methodology
ECLIA
Setup Schedule
Set Up:Daily Report Available:1 day
Reference Range
| THYROXINE, FREE: | 0.92-1.68 ng/dL |
Clinical Significance
Thyroxine (T4) circulates in the blood as an equilibrium mixture of free and serum protein bound hormone. Thyroxine binding globulin (TBG), albumin and pre-albumin bind approximately 75%, 10% and 15% of the total circulating T4 respectively. The binding of T4 by these proteins is such that less than 0.03% is present in the circulation as unbound, free T4 (FT4). This small percentage of the total T4 (TT4) represents the physiologically available hormone which is biologically active. Once the FT4 is absorbed by the target cells, the equilibrium reestablishes circulating FT4 levels. The equilibrium results in the maintenance of a constant level of FT4 when alterations occur in either the concentration or affinity of the serum binding proteins. Therefore, in a variety of normal (pregnancy) and abnormal (Familial Dysalbuminemic Hyperthyroxinemia, FDH) states, or as a result of the administration of certain drugs (e.g. furosemide and fenclofenac), the target tissues are assured of receiving the required amount of hormone. FT4 values may, therefore, provide the best indication of thyroid dysfunction, since FT4 is less sensitive to changes in the serum binding proteins.
Performing Laboratory
GBMC Chemistry
