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Laboratory Medicine

Test Directory / Free T4

Free T4

Brown clotted serum, gel barrier

TestFree T4
Common AbbreviationsFT4
ProfileThyroid Function Test (TFT)
Tube typeBrown clotted serum, gel barrier
Clinical IndicationFT4 is especially useful in the diagnostic differentiation between euthyroid, hyperthyroid and hypothyroid individuals. FT4 levels correlate with T4 secretion and metabolism. In hypothyroidism and hyperthyroidism, FT4 levels parallel changes in total T4 levels. Measuring free T4 is useful when altered levels of total T4 occur due to changes in T4 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, FT4 levels are unchanged, while total T4 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: 11 – 32 pmol/L 7 Days – 3 Months: 12 – 28 pmol/L 3 Months – 12 Months: 12 – 26 pmol/L 1 Year – 6 Years: 12 – 23 pmol/L 7 Years – 11 Years: 13 – 22 pmol/L 12 Years – 20 Years: 13 – 21 pmol/L Adults: 12 – 22 pmol/L 1st Trimester: 12 – 20 pmol/L 2nd Trimester: 10 – 17 pmol/L 3rd Trimester: 8 – 16 pmol/L (Quoted by the manufacturer)
LimitationsAnalysis should not be performed on haemolysed, icteric or lipaemic samples. The assay is unaffected by biotin < 81.8 nmol/L or < 20 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 Samples should not be taken from patients receiving therapy with high biotin doses (i.e. > 5 mg/day) until at least 8 hours following the last biotin administration. Any influence that might affect the binding behaviour of the binding proteins can alter the result of the fT4 tests (e.g. drugs, Non?Thyroid?Illness or patients suffering from Familial Dysalbuminemic Hyperthyroxinemia)). The test cannot be used in patients receiving treatment with lipid-lowering agents containing D?T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4 to 6 weeks to allow the physiological state to become re-established. Autoantibodies to thyroid hormones can interfere with the assay. In vitro tests were performed on 16 commonly used pharmaceuticals. No interference with the assay was found. In addition, special thyroid drugs were tested. No interference was found. In in vitro studies the drugs Furosemide and Levothyroxine caused elevated fT4 findings at the daily therapeutic dosage level. In rare cases, interference due to extremely high titres of antibodies to analyte specific antibodies, streptavidin or ruthenium can occur.
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