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Test ID: FT4
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Thyroxine Free
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Useful For
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To help in evaluating thyroid function and to diagnose thyroid diseases together with thyroid-stimulating hormone
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Method name and description
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Electrochemiluminescence immunoassay (ECLIA): Competition principle
Electrochemiluminescence immunoassay (ECLIA) intended for use on the cobas e 801 immunoassay analyzer. Patient specimen, T4‑specific antibody labeled with a ruthenium complex. After addition of biotinylated T4 and streptavidin-coated microparticles, the still-free binding sites of the labeled antibody become occupied, forming an antibody-hapten complex and the complex becomes bound to the solid phase via interaction of biotin and streptavidin. The reaction mixture is aspirated into the measuring cell where the microparticles are magnetically captured onto the surface of the electrode. Unbound substances are then removed with ProCell II M. Application of a voltage to the electrode then induces chemiluminescent emission which is measured by a photomultiplier. Results are determined via a calibration curve which is instrument specifically generated by 2‑point calibration and a master curve.
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Clinical information
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Thyroxine (T4) is the main thyroid hormone produced by the thyroid gland.Together with triiodothyronine (T3) it plays a vital role in regulating the body's metabolic rate, influencing the cardiovascular system, growth and bone metabolism, and are important for normal development of gonadal functions and nervous system. Free T4 (fT4) is the unbound and biologically active form which represents only 0.03 % of the total T4. It is a useful tool in clinical routine diagnostics for the assessment of the thyroid status. 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.
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Specimen type / Specimen volume / Specimen container
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Specimen type: Serum, Plasma
Minimum volume of sample: 1 mL
Serum: Plain tube (red or yellow top)
Plasma: Li‑heparin tube
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Collection instructions / Special Precautions / Timing of collection
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Collect blood by standard venipuncture techniques as per specimen requirements. When processing samples in primary tubes (sample collection systems), follow the instructions of the tube manufacturer.
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Storage and transport instructions
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Storage: 5 days at 20 – 25°C
7 days at 2 – 8°C
30 days at ‑20 °C (± 5 °C)
Transport: 2-25°C
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Specimen Rejection Criteria
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Grossly hemolyzed, icteric and lipemic samples, wrong collection container, insufficient sample and heat‑inactivated samples.
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Biological reference intervals and clinical decision values
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Age
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Reference interval (pmol/L)
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From
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To
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From
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To
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0 Days
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5 Days
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11.6
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32.2
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6 Days
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2 Months
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11.6
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28.3
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2 Months
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12 Months
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11.6
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25.7
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1 Year
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5 Years
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12.9
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23.2
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5 Years
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10 Years
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12.9
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21.9
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10 Years
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19 Years
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12.9
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20.6
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19 Years
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150 Years
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11.0
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23.3
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Interpretative comment:
For patient on thyroxine therapy, therapeutic levels for FT4 may be slightly higher than reference intervals of normal population. However, TSH should be the guide for maintenance of therapy.
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Factors affecting test performance and result interpretation
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Autoantibodies to thyroid hormones can interfere with the assay.
Drugs Furosemide and Levothyroxine caused elevated fT4 findings at the daily therapeutic dosage level.
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Turnaround time / Days and times test performed / Specimen retention time
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Daily (24/7)
Turn-around time:
Routine: One working day
Specimen Retention: 4 days
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