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Free T3 (FT3)

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EPIC Test Name

T3, FREE

EPIC Code

LAB137

Specimen Requirements

plasma
Minimum Volume:0.5 mL
Collection:Collect specimens using standard laboratory procedures.
Transport:Room Temperature ASAP
Stability:Room Temperature: 5 days at 20-25 degrees C
Refrigerated: 7 days at 2-8 degrees C
Frozen: 1 month at -20 degrees C
Container:LT GRN
Processing/Storage:Freeze only once
Rejection Causes:Hemolysis,
Insufficient Sample Volume

Methods

Electrochemiluminescence

Turnaround Time

SpecimenTurnaround TimeFrequency
plasmaRoutine: 4 hours24/7

Reference Ranges

Electrochemiluminescence
AgeAll RangeUnit
4 days up to 2 months2.00-5.20 pg.mLpg/mL
2 months up to 2 years1.50-6.40 pg/mL
2 years up to 7 years2.00-6.00 pg/mL
7 years up to 12 years2.70-5.20 pg/mL
12 years up to 20 years2.30-5.00 pg/mL
20 years up to 999 years2.00-4.40 pg/mL

Clinical Indications

T3 is largely produced by peripheral deiodination of T4, and a small amount of T3 is also directly secreted by the thyroid gland. T3 has a higher affinity than T4 for the thyroid hormone receptor of the target tissues and is considered to be the active hormone. Majority of both T3 and T4, the thyroid hormones, in the circulation are bound to the carrier proteins including thyroid-binding globulin (TBG), prealbumin, and albumin, while a small fraction circulates as unbound (free) form. Only approximately 0.3% of the total T3 exists as unbound or free form, the free fraction is the biologically active.
A reduction in the T4 to T3 conversion results in a decrease of T3 concentration. It can occur due to the side effects of certain medications, e.g., propranolol, glucocorticoids or amiodarone, or in patients with severe non-thyroidal illness (NTI).

Additional Information

• T3 is not considered as a sensitive marker as TSH for hypothyroidism.
• Certain drugs, e.g., phenytoin, phenylbutazone, and salicylates, can displace T3 from its binding proteins, leading to a reduction in the total T3 level, but no changes of free T3 levels. The study by Surks showed in the drug-treated patients, free T4 and T3 fractions were increased that offset the significant decrease in the total T4 and T3, resulting in normal free T4 and free T3 concentrations.
• Existence of autoantibodies to thyroid hormones can interfere with the immunoassay.
• T3 binding proteins’ anomalies or amount (pregnancy or severe liver diseases) may cause abnormal T3 results, although the patient may be in a euthyroid state. Free T3 or free T4, and TSH testing are indicated in these cases.

Performed

Lab
Chemistry - Downtown

Interpretative Information

• Elevated fT3 values are associated with thyrotoxicosis or excess thyroid hormone replacement.
• In hyperthyroidism, both T4 and T3 levels are usually elevated, but in patients with T3-toxicosis, only T3 levels are elevated, and reduced TSH levels.
• In hypothyroidism T4 and T3 levels are decreased. T3 levels are frequently low in sick or hospitalized euthyroid patients. For patients with subclinical hypothyroidism, T3 and T4 are within reference ranges, but with reduced TSH levels.
• In generally, fT3 measurement is not necessary since total T3 suffice. However, abnormal levels (high or low) of thyroid hormone-binding proteins, that can occur to clinically euthyroid patients, e.g., pregnancy, dysalbuminemia, may cause abnormal total T3 concentrations. In such cases, discordance between T4/T3 and TSH, but accordance between Free T4/Free T4 and TSH may occur.

CPT

84481

LOINC

3051-0

References

1. Pilo A, Lervasi G, Vitek F, et al.: Thyroidal and peripheral production of 3,5,3’-triiodothyronine in humans by multicompartmental analysis. Am J Physiol. 1990;258:E715-26. PMID 2333963
2. Schroeder A, Jimenez R, Young B, et al.: The ability of thyroid hormone receptors to sense t4 as an agonist depends on receptor isoform and on cellular cofactors. Mol Endocrinol. 2014;28:745-57. PMID 24673558
3. Franklyn JA, Wilkins MR, Wilkinson R, Ramsden DB, Sheppard MC. The effect of propranolol on circulating thyroid hormone measurements in thyrotoxic and euthyroid subjects. Acta Endocrinol (Copenh). 1985;108(3):351-5. doi: 10.1530/acta.0.1080351. PMID: 3920853.
4. Dong BJ. How medications affect thyroid function. West J Med. 2000;172(2):102-6. doi: 10.1136/ewjm.172.2.102. PMID: 10693372; PMCID: PMC1070767.
5. Surks MI, DeFesi CR. Normal serum free thyroid hormone concentrations in patients treated with phenytoin or carbamazepine. A paradox resolved. JAMA. 1996;275:1495-8. PMID: 8622224.
6. Hay ID, Klee GG: Linking medical needs and performance goals: clinical and laboratory perspectives on thyroid disease. Clin Chem. 1993;39:1519-1524
7. Klee GG: Clinical usage recommendations and analytic performance goals for total and free triiodothyronine measurements. Clin Chem. 1996;42:155-159
8. Freedman DB, Halsall D, Marshall WJ, Ellervik C: Thyroid disorders. In: Rifai N, Horvath AR, Wittwer CT: eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1572-1616

Contact Information

Chemistry - Downtown: (315)464-4460
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