|Test Id||Reporting Name||Available Separately||Always Performed|
|T4F||T4 (Thyroxine), Free, S||No||Yes|
|T4CC||T4 (Thyroxine), Total Only, S||No||Yes|
Electrochemiluminescence Immunoassay (ECLIA)
|Specimen Type||Temperature||Time||Special Container|
|Serum||Refrigerated (preferred)||7 days|
Thyroxine (T4) is synthesized in the thyroid gland. T4 is metabolized to triiodothyronine (T3) peripherally by deiodination. T4 is considered a reservoir or prohormone for T3, the biologically most active thyroid hormone. About 0.05% of circulating T4 is in the free or unbound portion. The remainder is bound to thyroxine-binding globulin), prealbumin, and albumin.
The hypothalamus secretes thyrotropin-releasing hormone), which stimulates the pituitary to release thyrotropin, formerly thyroid-stimulating hormone, (TSH). TSH stimulates the thyroid to secrete T4. T4 is partially converted peripherally to T3. High amounts of T4 and T3 (mostly from peripheral conversion of T4) cause hyperthyroidism.
T4 and T3 cause positive feedback to the pituitary and hypothalamus with resultant suppression or stimulation of the thyroid gland as follows: decrease of TSH if T3 or T4 is high (hyperthyroidism),and increase of TSH if T3 or T4 is low (hypothyroidism).
Measurement of total T4 gives a reliable reflection of clinical thyroid status in the absence of protein binding abnormalities. However, changes in binding proteins can occur which affect the level of total T4 but leave the level of unbound hormone unchanged.
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.
Elevations in FT4 cause hyperthyroidism, while decreases cause hypothyroidism.
T4 (THYROXINE), TOTAL ONLY
0-5 days: 5.0-18.5 mcg/dL
6 days-2 months: 5.4-17.0 mcg/dL
3 -11 months: 5.7-16.0 mcg/dL
1 -5 years: 6.0-14.7 mcg/dL
6 -10 years: 6.0-13.8 mcg/dL
11 -19 years: 5.9-13.2 mcg/dL
> or =20 years: 4.5-11.7 mcg/dL
T4 (THYROXINE), FREE
0-5 days: 0.9-2.5 ng/dL
6 days-2 months: 0.9-2.2 ng/dL
3-11 months: 0.9-2.0 ng/dL
1-5 years: 1.0-1.8 ng/dL
6-10 years: 1.0-1.7 ng/dL
11-19 years: 1.0-1.6 ng/dL
> or =20 years: 0.9-1.7 ng/dL
Melmed S, Polonsky KS, Larsen PR, et al: Williams Textbook of Endocrinology. 12th ed. Elsevier Saunders; 2011:348-414
Testing is performed on a Roche cobas instrument. The Roche thyroxine (T4) assay is a competitive assay using electrochemiluminescence detection. Bound T4 is released from binding proteins by 8-anilino-1-naphthalene sulfonic acid (ANS). Patient specimen is incubated with sheep polyclonal anti-T4 antibody labeled with ruthenium. Streptavidin-coated microparticles and biotinylated T4 are added for a second incubation during which the still free binding sites of the labeled antibody become occupied. The resulting immunocomplex becomes bound to the solid phase by interaction of biotin and streptavidin. The reaction mixture is aspirated into the measuring cell where the microparticles are magnetically captured onto the surface of the electrode. Unbound substances are then removed and application of a voltage to the electrode induces the electrochemiluminescent emission. This signal is measured against a calibration curve to determine patient results.(Package insert: Elecsys T4 . Roche Diagnostics; V2 English 03/2020
Testing is performed on a Roche cobas immunoassay analyzer. In the Roche free thyroxine (FT4) assay the determination of free thyroxine is made with the aid of a specific anti-T4 antibody labeled with a ruthenium complex. After addition of biotinylated T4 and streptavidin-coated microparticles, the still-free binding sites of the labeled antibody become occupied, with formation of an antibody-hapten complex. The entire complex becomes bound to the solid phase via interaction of biotin and streptavidin. The reaction mixture is aspirated into the measuring cell where the microparticles are magnetically captured onto the surface of the electrode. Unbound substances are then removed with ProCell. Application of a voltage to the electrode then induces chemiluminescent emission which is measured by a photomultiplier.(Package insert: Elecsys FT4II . Roche Diagnostics; V1 01/2019 )
Monday through Sunday
This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.
|Test Id||Test Order Name||Order LOINC Value|
|T4FT4||T4 (Thyroxine), Total and Free||90224-7|
|Result Id||Test Result Name||
Result LOINC Value
Applies only to results expressed in units of measure originally reported by the performing laboratory. These values do not apply to results that are converted to other units of measure.
|T4F||T4 (Thyroxine), Free, S||83122-2|
|T4CC||T4 (Thyroxine), Total Only, S||83119-8|