Thyroxine (T4), Free, Direct

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Category:

Turnaround Time: Within 1 day
CPT Code: 84439
Test Type: 0.8 mL Serum
Stability Time:

Temperature

Period

Room temperature

14 days

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3
Reference Range:

See table.1

Age

Range (ng/dL)

0 to 3 d

0.66−2.71

4 to 30 d

0.83−3.09

31 d to 12 m

0.48−2.34

13 m to 5 y

0.85−1.75

6 to 10 y

0.90−1.67

11 to 19 y

0.93−1.60

>19 y

0.82−1.77

Overview:

Free T4 may be indicated when binding globulin (TBG) problems are perceived, or when conventional test results seem inconsistent with clinical observations. It is normal in subjects with high thyroxine-binding globulin hormone binding who are euthyroid (ie, free thyroxine should be normal in nonthyroidal diseases). It should be normal in familial dysalbuminemic hyperthyroxinemia.

FT4 may be increased with radiologic contrast agents, propranolol, amiodarone, and heparin. It may be decreased with carbamazepine (Tegretol®). Free T4 is a small part of total T4. Increased free T4 levels may occur in subjects with nonthyroid diseases. Such elevations are described as transient.2 Low values were reported in patients with nonthyroidal illness.3 Discrepancies in free T4 levels between methods are recognized.4 Reliability problems continue to be discussed with the direct (analog) methods.5 Results of kits intended to serve in place of equilibrium dialysis technique may differ from the reference method.

1. Reference intervals for children and adults. Elecsys Thyroid Test. Roche Diagnostics; May 2005. PubMed 8595709

2. Borst GC, Eil C, Burman KD. Euthyroid hyperthyroxinemia. Ann Intern Med. 1983; 98(3):366-378. PubMed 6187257

3. Cooke RR, Pratt R. Thyroid function tests in acutely ill patients. Comparison of analogue based free thyroid hormone assays with free thyroxine index. Pathology. 1986; 18(1):94-97. PubMed 3725439

4. Gruhn JG, Barsano CP, Kumar Y. The development of tests of thyroid function. Arch Pathol Lab Med. 1987; 111(1):84-100. PubMed 3541847

5. Bethune JE. Interpretation of thyroid function tests. Dis Mon. 1989; 35(8):541-595. PubMed 2670494

Chattoraj SC, Watts NB. Endocrinology. In: Tietz NW, ed. Fundamentals of Clinical Chemistry. 3rd ed. Philadelphia, Pa: WB Saunders Co;1987: 533-613.

Gurnell M, Halsall DJ, Chatterjee VK. What should be done when thyroid function tests do not make sense? Clin Endocrinol (Oxf). 2011; 74(6):673-678. PubMed 21521292

Ingbar SH. Diseases of the thyroid. In: Braunwald E, Isselbacher KJ, Petersdorf RG, et al, eds. Harrison's Principles of Internal Medicine. 11th ed. New York, NY: McGraw-Hill; 1987: 1732-1752.

Jansson R, Forberg R, Levin K. Free thyroxin index and direct measurements of free thyroxin compared for evaluating postpartum autoimmune thyroid dysfunction. Clin Chem. 1984; 30(6):903-905. PubMed 6723049

Pearce CJ, Himsworth RL. Total and free thyroid hormone concentrations in patients receiving maintenance replacement treatment with thyroxine. Br Med J [Clin Res Ed]. 1984; 288(6418):693-695. PubMed 6421436

Surks MI, Chopra IJ, Mariash CN, Nicoloff JT, Solomon DH. American Thyroid Association guidelines for use of laboratory tests in thyroid disorders. JAMA. 1990 Mar 16; 263(11):1529-1532. PubMed 2308185

Wilkins TA. Free thyroxine assays: Analogue methods. Lancet. 1985; 2(8460):884. PubMed 2864591

Collection Details:

Patient Preparation:

Heparin has been reported to have in vivo and in vitro effects on free T4 assay. Hence samples should not be collected during or soon after the administration of this anticoagulant.

Collection Instructions:

This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

Red-top tube or gel-barrier tube.

If a red-top tube is used, transfer separated serum to a plastic transport tube.

Room temperature.