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Test Code LAB4174 T4 (Thyroxine), Total Only, Serum

Additional Codes

 

Test Name in EPIC EPIC Test Code Mnemonic Mayo Test ID
T4 (THYROXINE) TOTAL ONLY LAB4174 T4TOT T4

 

Reporting Name

T4 (Thyroxine), Total Only, S

Useful For

Monitoring treatment with synthetic hormones (synthetic triiodothyronine will cause a low total thyroxine: T4)

 

Monitoring treatment of hyperthyroidism with thiouracil and other antithyroid drugs

Testing Algorithm

See Thyroid Function Ordering Algorithm in Special Instructions.

Method Name

Electrochemiluminescence Immunoassay

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type

Serum


Specimen Required


Patient Preparation: For 12 hours before this blood test do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL


Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time
Serum Refrigerated (preferred) 7 days
  Frozen  30 days

Reject Due To

Hemolysis

Mild OK; Gross reject

Lipemia

Mild OK; Gross OK

Icterus

Mild OK; Gross OK

Other

NA

Reference Values

Adult (≥20 years): 4.5-11.7 mcg/dL

Pediatric:

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

Day(s) and Time(s) Performed

Monday through Friday 5 a.m. – 12 a.m., Saturday 6 a.m. – 6 p.m.

CPT Code Information

84436

LOINC Code Information

Test ID Test Order Name Order LOINC Value
T4 T4 (Thyroxine), Total Only, S 3026-2

 

Result ID Test Result Name Result LOINC Value
T4 T4 (Thyroxine), Total Only, S 3026-2

Interpretation

Values of more than 11.7 mcg/dL in adults or more than the age-related cutoffs in children are seen in hyperthyroidism and patients with acute thyroiditis.

 

Values below 4.5 mcg/dL in adults or below the age-related cutoffs in children are seen in hypothyroidism, myxedema, cretinism, chronic thyroiditis, and occasionally, subacute thyroiditis.

 

Increased total thyroxine (T4) is seen in pregnancy and patients who are on estrogen medication. These patients have increased total T4 levels (increased thyroxine-binding globulin: TBG levels).

 

Decreased total T4 is seen in patients on treatment with anabolic steroids, or nephrosis (decreased TBG levels).

 

A thyrotropin-releasing hormone stimulation test may be required for certain cases of hyperthyroidism.

 

Clinical findings are necessary to determine if thyroid-stimulating hormone, TBG, or free T4 testing is needed.

Cautions

In pregnancy, incomplete release of thyroxine (T4) from its binding proteins might result in falsely low total T4 levels. Therefore, total T4 should not be used as the only marker for thyroid function evaluation.

 

Thyroid-stimulating hormone (TSH) may be better than thyroxine (T4) as the initial test of thyroid status. TSH is elevated in primary hypothyroidism. TSH is low in primary hyperthyroidism.

 

Free T4 may more accurately measure the physiologic amount of T4.

 

Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.

 

This test cannot be used in patients receiving treatment with lipid-lowering agents containing dextrothyroxine unless therapy is discontinued for 4 to 6 weeks to allow the physiological state to become reestablished prior to testing.

 

Autoantibodies to thyroid hormones can interfere with testing.

 

Binding protein anomalies may cause values that deviate from the expected results. Pathological concentrations of binding proteins can lead to results outside the reference range, although the patient may be in a euthyroid state.

 

In rare cases, interference due to extremely high titers of antibodies to analyte-specific antibodies, ruthenium or streptavidin can occur.

Method Description

Testing is performed on a Roche cobas instrument. The Roche thyroxine assay (T4) 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: Roche Cobas. Roche Diagnostics, Indianapolis, IN)

Analytic Time

Same day/1 day

Specimen Retention Time

14 days

Clinical Information

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 (TBG), prealbumin, and albumin.

 

The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary to release 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 and nonthyroidal illness. However, changes in binding proteins can occur that affect the level of total T4, but leave the level of unbound hormone unchanged.

 

See Thyroid Function Ordering Algorithm in Special Instructions.

Test Classification

This test has been cleared or approved by the U.S. 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.