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Test ID: FRT4D T4 (Thyroxine), Free, Dialysis, Serum

Reporting Name

T4 (Thyroxine), Free by Dialysis, S

Useful For

Determining thyroid status of sick, hospitalized patients


Determining thyroid status of patients in whom abnormal binding proteins have been identified


Possibly useful in pediatric patients

Clinical 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% circulates 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.


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.

Report Available

3 to 8 days

Day(s) Performed

Monday, Wednesday, Thursday

Clinical Reference

1. De Brabandere VI, Hou P, Stockl D, et al: Isotope dilution-liquid chromatography/electrospray ionization-tandem mass spectrometry for the determination of serum thyroxine as a potential reference method. Rapid Commun Mass Spectrom. 1998;12:1099-1103

2. Jain R, Uy HL: Increase in serum free thyroxine levels related to intravenous heparin treatment. Ann Intern Med. 1996 Jan 1;124:74-75

3. Stockigt JR: Free thyroid hormone measurement. A critical appraisal. Clin Endocrinol Metab. 2001 Jun;30:265-289

4. Soldin SJ, Soukhova N, Janici N, Jonklaas J, Soldin OP: The measurement of free thyroxine by isotope dilution tandem mass spectrometry. Clin Chimica Acta. 2005;358:113-118

5. Sakai H, Nagao H, Sakurai M, et al: Correlation between serum levels of 3,3',5'-triiodothyronine and thyroid hormones measured by liquid chromatography-tandem mass spectrometry and immunoassay. PLoS One. 2015;10(10):e0138864 doi: 10.1371/journal.pone.0138864; Correction in: PLoS One. 2016;11(7):e0159169

6. Kahric-Janicic N, Soldin SJ, Soldin OP, West T, Gu J, Jonklaas J: Tandem mass spectrometry improves the accuracy of free thyroxine measurements during pregnancy.Thyroid. 2007 Apr;17(4):303-311 doi: 10.1089/thy.2006.0303

Method Name

Equilibrium Dialysis/Tandem Mass Spectrometry (MS/MS)

Specimen Type


Ordering Guidance

The routine free thyroxine test (FRT4 / T4 [Thyroxine], Free, Serum) is faster and provides useful information in most patients.

Necessary Information

Include name and telephone number of contact physician.

Specimen Required

Collection Container/Tube:

Preferred: Red top

Acceptable: Serum gel

Submission Container/Tube: Plastic vial

Specimen Volume: 2.6 mL

Collection Instructions:

1. Draw blood immediately before next scheduled dose.

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

Specimen Minimum Volume

1.2 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 28 days
  Frozen  21 days
  Ambient  7 days

Reference Values

0.8-2.0 ng/dL

Reference values apply to all ages.

Test Classification

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
FRT4D T4 (Thyroxine), Free by Dialysis, S 6892-4


Result ID Test Result Name Result LOINC Value
8859 T4 (Thyroxine), Free by Dialysis, S 6892-4
Mayo Clinic Laboratories | Endocrinology Catalog Additional Information: