T4, Free (FT4)

Alternative Names
  • Free T4 Direct Serum
  • Free Thyroxine T4 Free Direct Serum
  • Non-Protein Bound Tetraiodothyronine

Test Code: 4014

CPT: 84439
Tests Included Free T4
Use Both free and bound forms of T4 and T3 are present in the blood. More than 99% of the T4 and T3 circulate in the blood bound to carrier proteins, leaving less than 1% unbound. It is this level of unbound or free hormone that correlates with the functional thyroid state in most individuals.1,2
Clinical Utility Clearly elevated free T4 levels support the clinical diagnosis of hyperthyroidism while clearly low free T4 levels coupled with appropriate clinical findings, can establish a diagnosis of hypothyroidism. Measurement of free T4 levels along with other thyroid tests and clinical findings can establish borderline hyperthyroid and hypothyroid diagnoses.3

Low TSH, High/Normal T4 and T3, Hyperthyroidism
Low TSH & Low/Normal T4 & T3, Non-thyroidal illness; rare pituitary (secondary hypothyroidism
Normal TSH & High T4 & T3, Thryroid Hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid homrone High TSH, Normal T4 & T3, Mild (subclinical) hypothyroidism
High TSH, Low T4 & Low/Normal T3, Hypothyroidism
Intended Patient Population 18+ and Older Adult Males & Females
Patient Preparation None Specified
Sample Serum, Plasma
Tube Red, Green, Tiger
Volume 4mL Whole Blood (1mL Serum/Plasma)
Min Sample Volume 0.5 mLs
Reference Ranges M & F ≥ 18 yrs old; 0.8-1.8 ng/dL
Analytical Measurement Range 0.25-6.00 ng/dL
Critical Values FT4 (Free Thyroxine) < 50 yrs - ≥ 7.8 ng/dL 
FT4 (Free Thyroxine) ≥ 50 yrs - ≥ 6.0 ng/dL
Units ng/dL
Test Methodology Chemiluminescent Immunoassay
Test Turnaround Time 2 Days
Limitations Non-thyrometabolic disorders may cause abnormal free T4 levels. Anticonvulsant drug therapy (particularly phenytoin) may result in decreased free T4 levels due to an increased hepatic metabolism, and secondarily to displacement of hormone from binding sites.2,4,5 Anti-inflammatory drugs such as salicylate and phenylbutazone also compete for hormone binding sites, but their effect on free T4 levels has not been clearly defined.2,6

Patients on heparin therapy may have elevated free T4 levels due to release of non esterified fatty acids, which can alter the relationship between free and bound hormones.5 Determination of thyroid status in patients with non-thyroidal illness (NTI) should be interpreted with caution.2,7 In rare conditions, such as Familial Dysalbuminemic Hyperthyroxinemia (FDH), direct free hormone assays may yield erroneous results due to the extreme variations in the albumin-binding capacity for T4. Drugs such as Asprin, Sodium saliclate, and Phenylbutazone have also been shown to elevate results.
Shipping Requirements Refrigerated
Specimen Stability 7 Days RF
Reject Criteria Gross Hemolysis
Laboratory Developed Test (LDT) Yes
CMS Guidance NCD 190.22
References
  1. Gornall, AG, Luxton, AW, Bhavnani, BR. Endocrine disorders. In Applied Biochemistry of Clinical Disorders. 1986, 305-318. Philadelphia, PA: J. B. Lippincott Co.
  2. White, GH. Recent advances in routine thyroid function testing. CRC - Critical Reviews in Clinical Laboratory Sciences, 1987, 24: 315-362.
  3. Watts, NB, Keffer, JH. The thyroid gland. In Practical Endocrine Diagnosis. 1982; 77-96. Philadelphia, PA: Lea Febiger.
  4. Liewendahl, K, Majuri, H, Helenius, T. Thyroid function tests in patients on long-term treatment with various anticonvulsant drugs. Clinical Endocrinology, 1978; 8: 187-191.
  5. Wenzel, KW. Pharmacological interference with in vitro tests of thyroid function. Metabolis. 1981; 30(7): 717-732.
  6. Wilke, TJ. Estimation of free thyroid hormone concentrations in the clinical laboratory. Clinical Chemistry. 1986; 32(4): 585-592.
  7. Spencer, CA. Thyroid status: trends in testing - selective test use cuts cost. Clinical Chemistry News, November, 1989, 15(11): 9-14.

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