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23241 Thyroxine, Free by Dialysis (FRT4D)

Thyroxine, Free by Dialysis (FRT4D)
Test Code: MISC
Synonyms/Keywords
​Free T4 (Thyroxine), FT4 by Dialysis, FT4 by Equilibrium Dialysis, FT4 by Tandem Mass Spectrometry, T4 (Thyroxine) Free,
T4 by Dialysis, Thyroxine (T4), Total or Free, ZT4DIAL
Useful For
Determining thyroid status of sick, hospitalized patients
 
Used where abnormal binding proteins are known to exist
 
Possibly useful in pediatric patients - can be ordered as ZT4DIAL (all other providers should order as MISC).​
Specimen Requirements
Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
Serum​ ​Red Top Tube (RTT) ​Serum Separator Tube (SST) ​2.6 mL ​1.2 mL
Collection Processing Instructions

Include name and telephone number of contact physician.

1. Draw blood immediately before next scheduled dose.

2. Centrifuge and aliquot serum into plastic vial within 2 hours of draw.

Specimen Stability Information
Specimen Type Temperature Time
​Serum ​ ​ ​Refrigerated (preferred) ​28 days
​Frozen ​21 days
​Ambient​7 days
Rejection Criteria
Hemolysis
Gross
Lipemia​
Gross
Icterus​
Gross
Interference

Certain drugs may cause short-term free thyroxine fluctuations.

-Heparin

-Salicylates

  - Acetylsalicylic acid (aspirin)

  - Salicylic acid (salsalate)

-Furosemide

-Fenclofenac

-Mefenamic acid

-Flufenamic acid

-Diclofenac

-Diflunisal

-Phenytoin

-Carbamazepine

Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Mayo Clinic Laboratories ​Monday, Wednesday, Thursday ​3 days
Equilibrium Dialysis/Tandem Mass Spectrometry (MS/MS)​
Reference Lab
Test Information
Thyroxine (T4) and triiodothyronine (T3) are the 2 biologically active thyroid hormones. T4 makes up more than 80% of circulating thyroid hormones.
 
Following secretion by the thyroid gland, approximately 70% of circulating T4 and T3 are bound to thyroid-binding globulin (TBG), while 10% to 20% each are bound to transthyretin (TTR) and albumin, respectively. Less than 0.1% circulate as free T4 (FT4) or free T3 (FT3). FT4 and FT3 enter and leave cells freely by diffusion. Only the free hormones are biologically active, but bound and free fractions are in equilibrium. Equilibrium with TTR and albumin is rapid. By contrast, TBG binds thyroid hormones very tightly and equilibrium dissociation is slow. Biologically, TBG-bound thyroid hormone serves as a hormone reservoir and T4 serves as a prohormone for T3. Within cells, T4 is either converted to T3, which is about 5 times as potent as T4, or reverse T3, which is biologically inactive. Ultimately, T3, and to a much lesser degree T4, bind to the nuclear thyroid hormone receptor, altering gene expression patterns in a tissue-specific fashion.
 
Under normal physiologic conditions, FT4 and FT3 exert direct and indirect negative feedback on pituitary thyrotropin (thyroid stimulating hormone [TSH]) levels, the major hormone regulating thyroid gland activity. This results in tight regulation of thyroid hormone production and constant levels of FT4 and FT3 independent of the binding protein concentration. Measurement of FT4 and FT3, in conjunction with TSH measurement, therefore represents the best method to determine thyroid function status. It also allows determination of whether hyperthyroidism (increased FT4) or hypothyroidism (low FT4) are primary (the majority of cases, TSH altered in the opposite direction as FT4) or secondary/tertiary (hypothalamic/pituitary origin, TSH altered in the same direction as FT4). By contrast, total T4 and T3 levels can vary widely as a response to changes in binding protein levels, without any change in free thyroid hormone levels and, hence, actual thyroid function status.
 
FT4 is usually measured by automated analog immunoassays. In most instances, this will result in accurate results. However, abnormal types or quantities of binding proteins found in some patients and most often related to other illnesses or drug treatments, may interfere in the accurate measurement of FT4 by analog immunoassays. These problems can be overcome by measuring FT4 by equilibrium dialysis, free from interfering proteins.
Reference Range Information
0.8-2.0 ng/dL
Reference values apply to all ages.
Interpretation

​All free hormone assays should be combined with thyrotropin (thyroid-stimulating hormone) measurements.

Free thyroxine (FT4) levels below 0.8 ng/dL indicate possible hypothyroidism. FT4 levels above 2.0 ng/dL indicates possible hyperthyroidism.

Neonates can have significantly higher FT4 levels. The hypothalamic-pituitary-thyroid axis can take several days or, sometimes, weeks to mature.

Outreach CPTs
CPT Modifier
(if needed)
Quantity Description Comments
​84439
Synonyms/Keywords
​Free T4 (Thyroxine), FT4 by Dialysis, FT4 by Equilibrium Dialysis, FT4 by Tandem Mass Spectrometry, T4 (Thyroxine) Free,
T4 by Dialysis, Thyroxine (T4), Total or Free, ZT4DIAL
Ordering Applications
Ordering Application Description
If the ordering application you are looking for is not listed, contact your local laboratory for assistance.
Specimen Requirements
Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
Serum​ ​Red Top Tube (RTT) ​Serum Separator Tube (SST) ​2.6 mL ​1.2 mL
Collection Processing

Include name and telephone number of contact physician.

1. Draw blood immediately before next scheduled dose.

2. Centrifuge and aliquot serum into plastic vial within 2 hours of draw.

Specimen Stability Information
Specimen Type Temperature Time
​Serum ​ ​ ​Refrigerated (preferred) ​28 days
​Frozen ​21 days
​Ambient​7 days
Rejection Criteria
Hemolysis
Gross
Lipemia​
Gross
Icterus​
Gross
Interference

Certain drugs may cause short-term free thyroxine fluctuations.

-Heparin

-Salicylates

  - Acetylsalicylic acid (aspirin)

  - Salicylic acid (salsalate)

-Furosemide

-Fenclofenac

-Mefenamic acid

-Flufenamic acid

-Diclofenac

-Diflunisal

-Phenytoin

-Carbamazepine

Useful For
Determining thyroid status of sick, hospitalized patients
 
Used where abnormal binding proteins are known to exist
 
Possibly useful in pediatric patients - can be ordered as ZT4DIAL (all other providers should order as MISC).​
Reference Range Information
0.8-2.0 ng/dL
Reference values apply to all ages.
Interpretation

​All free hormone assays should be combined with thyrotropin (thyroid-stimulating hormone) measurements.

Free thyroxine (FT4) levels below 0.8 ng/dL indicate possible hypothyroidism. FT4 levels above 2.0 ng/dL indicates possible hyperthyroidism.

Neonates can have significantly higher FT4 levels. The hypothalamic-pituitary-thyroid axis can take several days or, sometimes, weeks to mature.

For more information visit:
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Mayo Clinic Laboratories ​Monday, Wednesday, Thursday ​3 days
Equilibrium Dialysis/Tandem Mass Spectrometry (MS/MS)​
Reference Lab
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.