Thyroxine (T4), Free, Direct

CPT: 84439
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  • Free T4, Direct, Serum
  • Free Thyroxine
  • T4, Free, Direct, Serum

Special 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.

Expected Turnaround Time

Within 1 day

Related Documents

For more information, please view the literature below.

Thyroid Testing: Assessing Thyroid Disease in Your Patients

Specimen Requirements


Serum (preferred) or plasma


1 mL

Minimum Volume

0.7 mL (Note: This volume does not allow for repeat testing.)


Red-top tube, gel-barrier tube, or green-top (lithium heparin) tube. Do not use oxalate, EDTA, or citrate plasma.


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

Storage Instructions

Room temperature

Stability Requirements



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

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.

Causes for Rejection

Citrate plasma specimen; improper labeling

Test Details


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.


Electrochemiluminescence immunoassay (ECLIA)

Reference Interval

See table.1


Range (ng/dL)

0 to 3 d


4 to 30 d


31 d to 12 m


13 m to 5 y


6 to 10 y


11 to 19 y


>19 y



1. Reference intervals for children and adults. Elecsys Thyroid Test. Roche Diagnostics; May 2005. 8595709
2. Borst GC, Eil C, Burman KD. Euthyroid hyperthyroxinemia. Ann Intern Med. 1983; 98(3):366-378. 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. 3725439
4. Gruhn JG, Barsano CP, Kumar Y. The development of tests of thyroid function. Arch Pathol Lab Med. 1987; 111(1):84-100. 3541847
5. Bethune JE. Interpretation of thyroid function tests. Dis Mon. 1989; 35(8):541-595.2670494


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):673-678.21521292
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. 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. 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. 2308185
Wilkins TA. Free thyroxine assays: Analogue methods. Lancet. 1985; 2(8460):884.2864591


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
001974 Thyroxine (T4) Free, Direct 3024-7 019745 T4,Free(Direct) ng/dL 3024-7

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