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T4, FREE

Item Value
Available Stat? No
Test code FT4
Performed by? Chemistry
Sendout? no
Price range $$
In House Availability Monday - Friday
Principle The Siemens Centaur assay is a competitive immunoassay using direct chemiluminescent technology. Free T4 competes with labeled T4 for a limited amount of polyclonal rabbit anti-T4 antibody bound to a solid phase. This is a direct one-step analog assay for free T4.


Interpretation Circulating free thyroxine (FT4), which is only 1/2000 of the total thyroxine, is the metabolically active fraction. Hyperthyroid and hypothyroid patients tend to have greater alteration of FT4 than of total serum T4; the latter is highly dependent upon the plasma concentration of thyroxine binding proteins. Low and high values are associated with hypo- and hyperthyroidism, respectively, but may also occur in severe illness or with certain drugs such as heparin, salicylate, phenytoin, cholecystographic agents (iopanoic acid and ipodate), amiodarone, and propranolol. FT4 may be normal in T3 thyrotoxicosis.
Container type gold top gel tube
Amount to Collect 2 mL
Collection Instructions Plasma samples will not be assayed.
Sample type Blood
Normal range
Age Free T4 (ng/dL)
0-1 month 0.66 – 2.37
1-12 months 0.71 – 1.96
1-5 years 0.81 – 1.74
6-10 years 0.82 – 1.62
11-14 years 0.81 – 1.66
15-18 years 0.82 – 1.76
> 18 years 0.80 – 1.76

Synonyms Free T4;FT4;Thyroxine, Free (FT4);
Stability Serum is stable 48 hrs at 2-8°C, 30 days at -20°C.
Interferences 1. Patients taking phenytoin and carbamazepine may have decreased free T4 values.

2. One-step, analog methods for free T4 may produce falsely low or falsely high values in patients with congenital thyroxine-binding globulin (TBG) excess or deficiency, dysalbunemic hyperthyroidism, circulating T4 antibodies or severe nonthyroidal illness. Consult the Laboratory Medicine Resident in Clinical Chemistry (x65527, pager 415-443-2311) if there are questions about FT3, FT4 or TSH results. Additionally, in pregnant women there is an increase in TBG that may cause abnormal FT4 results using this methodology.

3. Auto- (rheumatoid factor) and heterophile antibodies in human serum can react with reagent immunoglobulins, interfering with in vitro immunoassays. Patients routinely exposed to animals or to animal serum products can be prone to this interference. Anomalous values, spuriously low or high, may be observed. Results inconsistent with clinical findings or previous laboratory values can be investigated by the laboratory for this phenomenon. Call the Clinical Chemistry Laboratory Medicine Resident (x65527, pager 415-443-2311).

References 1. Bayer 111624 Rev. J, 2002-07. Siemens Medical Solutions Diagnostics, Malvern, PA.

2. Lee, R.H., Spencer, C.A., Mestman, J.H., Miller, E.A., Petrovic, I., Braverman, L.E., and Goodwin, T.M. (2009) Free T4 immunoassays are flawed during pregnancy. 200(3):260e1-260e6. American Journal of Obstetrics and Gynecology.

3. Fritz, K.S., Wilcox, R.B., and Nelson, J.C. (2007) Quantifying spurious free T4 results attributable to thyroxine-binding proteins in serum dialysates and ultrafiltrates. 53: 985-988. Clinical Chemistry.

4. Kapelari K, Kirchlechner C, Hšgler W, Schweitzer K, Virgolini I, Moncayo R. Pediatric reference intervals for thyroid hormone levels from birth to adulthood: a retrospective study. BMC Endocr Disord 8:15, 2008.
CPT coding 84439
Last Updated 7/15/2011 11:43:07 AM
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