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Laboratory Medicine

Test Directory / Free T3

Free T3

Brown clotted serum, gel barrier

TestFree T3
Common AbbreviationsFT3
Tube typeBrown clotted serum, gel barrier
Clinical IndicationFT3 is especially useful in the diagnostic differentiation between euthyroid, hyperthyroid and hypothyroid individuals.FT3 levels correlate with T3 secretion and metabolism. In hypothyroidism and hyperthyroidism, FT3 levels parallel changes in total T3 levels. Measuring free T3 is useful when altered levels of total T3 occur due to changes in T3 binding proteins, especially TBG. TBG levels remain relatively constant in healthy individuals, but certain conditions, such as pregnancy and steroid therapy, can alter these levels. In these conditions, FT3 levels are unchanged, while total T3 levels parallel the changes in TBG.
Specimen TypeBlood
Sample typeSerum
Minimum Volume0.5mL If requesting more than 10 tests please send an additional brown clotted serum sample.
Special PrecautionsNo special requirements
Stability5 days at 20 - 25°C, 7 days at 2 - 8°C and 30 days at -20°C
Turnaround TimeInpatient: 24 hours Outpatient/ GP: 24 hours
LaboratoryYork and Scarborough
Reference Interval0 - 6 Days: 2.7 - 9.7 pmol/L 7 Days - 3 Months: 3.0 - 9.3 pmol/L 3 Months -12 Months: 3.3 -9.0 pmol/L 1 Year – 6 Years: 3.7 – 8.5 pmol/L 7 Years – 11 Years: 3.9 – 8.0 pmol/L 12 Years – 20 Years: 3.9 – 7.7 pmol/L Adults: 3.1 – 6.8 pmol/L 1st Trimester: 3.8 – 6.0pmol/L 2nd Trimester: 3.2 – 5.5 pmol/L 3rd Trimester: 3.1 – 5.0 pmol/L (Quoted by the manufacturer)
LimitationsAnalysis should not be performed on haemolysed, icteric or lipaemic samples. The assay is unaffected by biotin < 4912 nmol/L or < 1200 ng/mL. No interference was observed from rheumatoid factors up to a concentration of 1200 IU/mL. No interference was observed from IgG < 7.0 g/dL; IgA < 1.6 g/dL; IgM < 1.0 g/dL In vitro tests were performed on 17 commonly used pharmaceuticals. No interference with the assay was found. In addition, special thyroid drugs were tested. No interference was found. Any influence that might affect the binding behaviour of the binding proteins can alter the result of the fT3 tests (e.g. drugs, Non-Thyroid Illness or patients suffering from Familial Dysalbuminemic Hyperthyroxinemia). In rare cases, interference due to extremely high titres of antibodies to analyte specific antibodies, streptavidin or ruthenium can occur.
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