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HelpTest Code TT4 Total T4
Additional Codes
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Cerner |
NextGen |
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Total T4 |
Total T4 |
Alternate Name(s)
TT4
Total Thyroxine
Total Tetraiodothyronine
Methodology
Competitive Immunoassay
Patient Preparation
None
Collection Instructions
Standard phlebotomy practices.
Specimen Requirements
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Container |
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Stability |
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Rejection Criteria |
Gross Hemolysis Turbid Specimens |
Result Reporting and Reference Values
Reported in μg/dL.
Pediatric refence ranges have not been established.
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Age |
Low |
High |
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18-150 Years |
5.53 |
11.0 |
Reflex Testing
None
Limitations
- Ortho reports a bias with the following:
- D thyroxine and L-3, 3, 5-Triiodothyroacetic acid (TRIAC).
- Heterophilic antibodies in serum or plasma samples may cause interference in immunoassays. These antibodies maybe present in blood samples from individuals regularly exposed to animals or who have been treated with animal serum products. Results that are inconsistent with clinical observations indicate the need for additional testing.
- Thyroid hormone autoantibodies in samples may cause interference with this test. Results that are inconsistent with clinical observations indicate the need for additional testing.
- Certain drugs and clinical conditions are known to alter TT4 concentrations in vivo. For additional information, refer to one of the published summaries.
- Young DS. Effects of Drugs on Clinical Laboratory Tests ed. 4. Washington, D.C.: AACC Press; 1995.
- Friedman RB, Young DS. Effects of Disease on Clinical Laboratory Tests. ed. 3. Washington, D.C.: AACC Press; 1997.
- Tryding N, Tufvesson C, Sonntag O (eds). Drug Effects in Clinical Chemistry. ed. 7. Stockholm: The National Corporation of Swedish Pharmacies, Pharmasoft AB, Swedish Society for Clinical Chemistry; 1996.
Useful For
Thyroxine accounts for at least 90% of circulating protein-bound iodine. While >99.9% of T4 is protein-bound, primarily to thyroxine-binding globulin (TBG), it is the free fraction that is biologically active. In most patients the total T4 level is a good indicator of thyroid status, however it can sometimes be inadequate, and diagnostic efficiency may be improved by use of a total T4 test in conjunction with other tests. T4 levels may be altered in conditions affecting the capacity of the thyroid hormone binding proteins, e.g., pregnancy, when a test for triiodothyronine (T3) or T4 uptake may be used with the total T4 result to calculate the free T4 index (FT4I) to estimate the level of metabolically active free T4.