Thyroxine (T<sub>4</sub>) Free, Direct, Serum
| Thyroxine (T4) Free, Direct, Serum | | | |
| Number | | 001974 |
| CPT | | 84439 |
| Synonyms | | Free T4, Direct, Serum ; Free Thyroxine ; T4, Free, Direct ; T4, Free, Direct, Serum ; Unbound T4 |
| Specimen | | Serum |
| Volume | | 1 mL |
| Minimum Volume | | 0.3 mL (Note: This volume does not allow for repeat testing.) |
| Container | | Red-top tube or gel-barrier tube |
| Collection | | If a red-top tube is used, transfer separated serum to a plastic transport tube. |
| Storage Instructions | | Refrigerate |
| 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 | | Plasma specimen; gross lipemia |
| Reference Interval | | See table.1
| Age | Male
(ng/dL) | Female
(ng/dL) | | 1-3 d | 0.80-2.78 | 0.88-1.93 | | 4-30 d | 0.48-2.32 | 0.61-1.93 | | 1-11 mo | 0.76-2.00 | 0.88-1.84 | | 1-5 y | 0.90-1.59 | 1.02-1.72 | | 6-10 y | 0.81-1.68 | 0.82-1.58 | | 11-15 y | 0.92-1.57 | 0.79-1.49 | | 16-18 y | 0.92-1.53 | 0.83-1.44 | | >18 y | 0.70-1.53 | 0.61-1.76 |
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| Use | | 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. |
| Limitations | | 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. |
| Methodology | | Immunochemiluminometric assay (ICMA) |
| Footnotes | | - Soldin SJ, Morales A, Albalos F, et al, “Pediatric Reference Ranges on the Abbott IMx for FSH, LH, Prolactin, TSH, T4, T3, Free T4, Free T3, T-Uptake, IgE, and Ferritin,” Clin Biochem, 1995, 28(6):603-6.
- Borst GC, Eil C, and Burman KD, “Euthyroid Hyperthyroxinemia,” Ann Intern Med, 1983, 98(3):366-78 (review).
- Cooke RR and 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):94-7.
- Gruhn JG, Barsano CP, and Kumar Y, “The Development of Tests of Thyroid Function,” Arch Pathol Lab Med, 1987, 111(1):84-100.
- Bethune JE, “Interpretation of Thyroid Function Tests,” Dis Mon, 1989, 35(8):541-95
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| References | | Chattoraj SC and Watts NB, “Endocrinology,” Fundamentals of Clinical Chemistry, 3rd ed, Tietz NW, ed, Philadelphia, PA: WB Saunders Co, 1987, 533-613. Ingbar SH, “Diseases of the Thyroid,” Harrison's Principles of Internal Medicine, 11th ed, Braunwald E, Isselbacher KJ, Petersdorf RG, et al, eds, New York, NY: McGraw-Hill, 1987, 1732-52. Jansson R, Forberg R, and Levin K, “Free Thyroxin Index and Direct Measurements of Free Thyroxin Compared for Evaluating Postpartum Autoimmune Thyroid Dysfunction,” Clin Chem, 1984, 30(6):903-5. Pearce CJ and 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):693-5. Surks MI, Chopra IJ, Mariash CN, et al, “American Thyroid Association Guidelines for Use of Laboratory Tests in Thyroid Disorders,” JAMA, 1990, 263(11):1529-32. Wilkins TA, “Free Thyroxine Assays: Analogue Methods,” Lancet, 1985, 2(8460):884. |
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