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23091 Thyroxine, Free

Thyroxine, Free
Test Code: T4-FREE
Synonyms/Keywords
Free-T4, Free thyroxine, FT4
Useful For
Free-T4 generally is used as a second-line test to evaluate situations in which TSH abnormalities are found.
Specimen Requirements
Fasting Required Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)		 Pediatric Minimum Volume
(no repeat)
No​ Serum or Plasma​

Serum Separator Tube (SST)
Red Top Tube (RTT)​,
Lithium-heparin Plasma Separator Tube (PST)​		 0.5 mL​ 0.4 mL​ ​0.4mL
Collection Processing Instructions
Separate serum or plasma from the blood within 60 minutes of venipuncture. Specimen must be free of particulate matter including fibrin. Specimens collected in the RTT must be removed from the cells within one hour for storage or transport. Specimens collected in gel-barrier tubes must be removed from the primary tube prior to transporting to Marshfield. Do not send primary collection tube. Specimen should be transported refrigerated (2 - 8°C). If sent frozen, specimens must be completely thawed, thoroughly mixed, and centrifuged before analysis. 1 freeze/thaw cycle has shown to have no qualitative difference in results.
Specimen Stability Information
Specimen Type Temperature Time
Serum/Plasma​ ​ Ambient​ 8 hours​
Refrigerated​ 7 days
Frozen​ at -20 deg Celsius 30 days
Rejection Criteria
 
​Plasma samples in incorrect anticoagulant
Specimen not separated from cells within 60 minutes ​
Specimen not transported refrigerated​
Grossly hemolyzed​
 
Interference
Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating anti-animal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.
 
High doses of exogenous biotin (also termed Vitamin B7, Vitamin H or Coenzyme R) may interfere with this assay. It is recommended that patients refrain from consuming any multivitamin or supplement containing biotin for at least 72 hours prior to collection of a blood sample.
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Diagnostic Treatment Center ​Monday through Sunday​ ​Less than 2 hours​ ​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension
​Eau Claire ​Monday through Sunday​ ​Less than 2 hours​ ​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension
​Lakeview Medical Center ​Monday through Sunday​ ​Less than 2 hours​ ​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension
Marshfield​ Monday through Sunday​ Less than 2 hours​
Competitive Immunoassay using Direct Chemiluminescent Technology/Siemens Centaur
​Neillsville​Monday through Sunday​Less than 2 hours​​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension​

​Minocqua
​Monday through Sunday
​Less than 2 hours
Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension​​
Test Information

Free thyroxine comprises a small fraction of total thyroxine. The free T4 (FT4) is available to the tissues and is, therefore, the metabolically active fraction. Free thyroxine (FT4) are most commonly used in preference to total hormone measurements (TT4 or TT3) to improve the diagnostic accuracy for detecting hypo- and hyperthyroidism in patient populations with the thyroid hormone binding abnormalities.

The exception to this general rule occurs in those select conditions where TSH may be diagnostically unreliable in which case FT4 becomes the first-line test of choice. Such conditions include periods of unstable thyroid status (the early phase of treating hypo- or hyperthyroidism), when hypothalamic-pituitary dysfunction is suspected to be present, or when patients are taking drugs such as glucocorticoids that are known to affect TSH secretion.

Current FT4 testing methods have not yet received adequate validation in the hospital setting in which a wide variety of non-thyroidal illnesses and drug therapies are frequently encountered that are known to impair the diagnostic accuracy of both thyroid hormone and TSH testing.

Discordances between TSH and FT4 measured on the same specimen can on occasion cause a diagnostic dilemma. However, it should be recognized that the intrinsic log/linear TSH/FT4 relationship dictates that modest reductions in TSH (0.05 – 0.3 mIU/L), or modest elevations (3 – 10 mIU/L), would not be expected to be associated with changes in FT4 values outside the normal reference range.

Pregnancy: Non-pregnant FT4 reference ranges do not apply to pregnancy. Specifically low FT4 immunoassay values are observed in a significant proportion of women by the third trimester of pregnancy. Albumin levels tend to fall during pregnancy and the frequency of low FT4 values during pregnancy appears to be method-related, likely reflecting the albumin-dependence of the method.

Discrepancies in free T4 levels between methods are recognized.

Reference Range Information
Performing Location Reference Range
All Performing Sites using Dimension Analyzer​ ​0.7-1.4 ng/dL
Marshfield​
<1 month:         No reference range established          
1 mo-12 years:  0.9-1.4 ng/dL
13-20 years:      0.8-1.4 ng/dL
>20 years:         0.9-1.8 ng/dL
Interpretation

Free T4 results should be interpreted in light of the total clinical presentation of the patient, including: symptoms, clinical history, data from additional tests and other appropriate information.

Elevated values suggest hyperthyroidism or exogenous thyroxine. Decreased values suggest hypothyroidism. 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. Lithium and iodide preparations lower FT4 levels. FT4 may be increased with radiologic contrast agents, propranolol, amiodarone.

Patients on heparin therapy may have elevated free T4 levels due to release of non-esterified fatty acids, this can alter the relationship between free and bound hormones.

Outreach CPTs
CPT Modifier
(if needed)		 Quantity Description Comments
84439​ ​ ​ ​ ​
Synonyms/Keywords
Free-T4, Free thyroxine, FT4
Ordering Applications
Ordering Application Description
​Centricity ​T4-Free
​Cerner ​Thyroxine Free
​COM ​T4, Free
If the ordering application you are looking for is not listed, contact your local laboratory for assistance.
Specimen Requirements
Fasting Required Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)		 Pediatric Minimum Volume
(no repeat)
No​ Serum or Plasma​

Serum Separator Tube (SST)
Red Top Tube (RTT)​,
Lithium-heparin Plasma Separator Tube (PST)​		 0.5 mL​ 0.4 mL​ ​0.4mL
Collection Processing
Separate serum or plasma from the blood within 60 minutes of venipuncture. Specimen must be free of particulate matter including fibrin. Specimens collected in the RTT must be removed from the cells within one hour for storage or transport. Specimens collected in gel-barrier tubes must be removed from the primary tube prior to transporting to Marshfield. Do not send primary collection tube. Specimen should be transported refrigerated (2 - 8°C). If sent frozen, specimens must be completely thawed, thoroughly mixed, and centrifuged before analysis. 1 freeze/thaw cycle has shown to have no qualitative difference in results.
Specimen Stability Information
Specimen Type Temperature Time
Serum/Plasma​ ​ Ambient​ 8 hours​
Refrigerated​ 7 days
Frozen​ at -20 deg Celsius 30 days
Rejection Criteria
 
​Plasma samples in incorrect anticoagulant
Specimen not separated from cells within 60 minutes ​
Specimen not transported refrigerated​
Grossly hemolyzed​
 
Interference
Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating anti-animal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.
 
High doses of exogenous biotin (also termed Vitamin B7, Vitamin H or Coenzyme R) may interfere with this assay. It is recommended that patients refrain from consuming any multivitamin or supplement containing biotin for at least 72 hours prior to collection of a blood sample.
Useful For
Free-T4 generally is used as a second-line test to evaluate situations in which TSH abnormalities are found.
Reference Range Information
Performing Location Reference Range
All Performing Sites using Dimension Analyzer​ ​0.7-1.4 ng/dL
Marshfield​
<1 month:         No reference range established          
1 mo-12 years:  0.9-1.4 ng/dL
13-20 years:      0.8-1.4 ng/dL
>20 years:         0.9-1.8 ng/dL
Interpretation

Free T4 results should be interpreted in light of the total clinical presentation of the patient, including: symptoms, clinical history, data from additional tests and other appropriate information.

Elevated values suggest hyperthyroidism or exogenous thyroxine. Decreased values suggest hypothyroidism. 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. Lithium and iodide preparations lower FT4 levels. FT4 may be increased with radiologic contrast agents, propranolol, amiodarone.

Patients on heparin therapy may have elevated free T4 levels due to release of non-esterified fatty acids, this can alter the relationship between free and bound hormones.

For more information visit:
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Diagnostic Treatment Center ​Monday through Sunday​ ​Less than 2 hours​ ​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension
​Eau Claire ​Monday through Sunday​ ​Less than 2 hours​ ​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension
​Lakeview Medical Center ​Monday through Sunday​ ​Less than 2 hours​ ​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension
Marshfield​ Monday through Sunday​ Less than 2 hours​
Competitive Immunoassay using Direct Chemiluminescent Technology/Siemens Centaur
​Neillsville​Monday through Sunday​Less than 2 hours​​Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension​

​Minocqua
​Monday through Sunday
​Less than 2 hours
Homogeneous Sandwich Chemiluminescent Immunoassay (LOCI)/Siemens Dimension​​
For billing questions, see Contacts
Outreach CPTs
CPT Modifier
(if needed)		 Quantity Description Comments
84439​ ​ ​ ​ ​
For most current information refer to the Marshfield Laboratory online reference manual.