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HelpCatalog ID T4 T4, Total
CPOE Test Name
Thyroxine Total; T4 Total
Synonyms
Thyroxine; Tetraiodothyronine; T4
Specimen Type
Serum
Specimen Requirements
5 mL gold top SST tube, 2 mL blood minimum
Storage and Transport
Specimens may be stored at 2-8C for up to 24 hours. Specimens in which testing will be delayed for greater than 24 hours should be frozen.
Reference Values
4.5-12.0 mcg/dL
Interpretation
Elevated T4 levels are seen in hyperthyroidism. T4 is also easily impacted by changes in TBG levels. Consequently, T4 levels may be elevated with increased concentration of TBG, such as in pregnancy, administration of oral contraceptives or estrogen, infectious and chronic active hepatitis, biliary cirrhosis or congenital increase in TBG levels.
Decreased T4 levels are seen in hypothyroidism. T4 will also be reduced when TBG levels are decreased. Examples of situations which decrease TBG levels include nephrotic syndrome, androgen therapy, glucocorticoid therapy, major systemic illness or congenital decrease of TBG.
T4 values should be normalized for individual variations in thyroxine binding protein capacity. The FTI (Free Thyroxine Index) is available for this purpose.
Causes for Rejection
Lack of adequate labeling, insufficient specimen quantity, incorrect specimen type
Turn Around Time
12 hours from time of testing
CPT / HCPCS Code
84436
Performing Lab
Core Laboratory AD 3301 721-2120
Test Consists
The quantitative determination of Total T4 in serum or plasma.