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THYROXINE (T4) FREE, DIRECT, S
Message"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.
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Test Code
1977
Alias/See Also
"Free T4, Direct, Serum
Free Thyroxine
T4, Free, Direct, Serum
Unbound T4
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Free Thyroxine
T4, Free, Direct, Serum
Unbound T4
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CPT Codes
84439
Preferred Specimen
"Serum
0.8 mL
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0.8 mL
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Minimum Volume
"0.3 mL (Note: This volume does not allow for repeat testing.)"
Instructions
"If a red-top tube is used, transfer separated serum to a plastic transport tube.
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Transport Container
"Red-top tube or gel-barrier tube
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Transport Temperature
Refrigerated
Specimen Stability
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Room temperature
14 days
Refrigerated
14 days
Frozen
14 days
Freeze/thaw cycles
Stable x3
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Room temperature
14 days
Refrigerated
14 days
Frozen
14 days
Freeze/thaw cycles
Stable x3
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Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
"Citrate plasma specimen; improper labeling
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Methodology
"Electrochemiluminescence immunoassay (ECLIA) "
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."
Reference Range
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Age------------(ng/dL)
0-3 d---------0.66-2.71
4-30 d-----------0.83-3.09
31 d to 12 m--------0.48-2.34
13 m to 5 y-------0.85-1.75
6-10 y-------0.90-1.67
11-19 y-------0.93-1.60
>19 y------0.82-1.77
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Age------------(ng/dL)
0-3 d---------0.66-2.71
4-30 d-----------0.83-3.09
31 d to 12 m--------0.48-2.34
13 m to 5 y-------0.85-1.75
6-10 y-------0.90-1.67
11-19 y-------0.93-1.60
>19 y------0.82-1.77
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Clinical Significance
"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.
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