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T3, FREE

Item Value
Available Stat? No
Test code FT3
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 T3 in the sample competes with a T3 analog, bound to a solid phase, for a limited amount of labeled monoclonal mouse anti-T3 antibodies. This is a direct, one-step analog assay for free T3.
Interpretation Triiodothyronine (T3) and thyroxine (T4) are the two principal thyroid hormones. Both are synthesized by the thyroid gland and secreted into the bloodstream under stimulation by thyroid stimulating hormone (TSH).

Both T3 and T4 are physiologically important as either hormone can maintain the euthyroid state.

The majority of circulating T3 and T4 are bound to Thyroxine-binding globulin (TBG) and, to a lesser extent, to albumin and Thyroxine-binding prealbumin. Only the small unbound fractions of T3 (Free T3) and T4 (Free T4) are metabolically active. Measurement of Free T3 and Total T3 are useful in the diagnosis and monitoring of hyperthyroidism. T3 levels in serum have been recognized as being more sensitive than T4 levels for certain thyroid conditions.

In hyperthyroidism, T3 and T4 concentrations generally increase and usually correlate well with one another. However, in a condition known as T3 thyrotoxicosis, only T3 levels are increased.

The final definition of patient thyroid status should never depend upon the results of any single test, but should be determined in conjunction with one or more of other recommended standard thyroid test and, principally, through the clinical evaluation of the patient by the physician.
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 T3 (pg/mL)
0-1 month 3.0 – 6.0
1-12 months 2.8 – 5.0
1-5 years 2.6 – 5.1
6-10 years 2.6 – 4.9
11-18 years 2.6 – 4.8
> 18 years 2.3 – 4.2


Synonyms Free T3;Free Triiodothyronine (FT3);FT3;Triiodothyronine, Free (FT3);
Stability Serum is stable 48 hrs at 2-8°C, 60 days at -20°C
Interferences 1. One-step analog methods for free T3 may produce falsely low or falsely high results in some patients who have moderate to severe abnormalities in albumin and thyroid binding globulin (TBG), or who are critically ill (severe non-thyroidal illness). Consult the Laboratory Medicine Resident in Clinical Chemistry (x65527, pager 415-443-2311) if there are questions about free T3, free T4 and TSH results.

2. 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 117573 Rev. C, 5/2000. Siemens Medical Solutions Diagnostics, Malvern, PA.

2. 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 84481
Last Updated 7/15/2011 11:49:24 AM
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