Test ID: T4
T4 (Thyroxine), Total Only, Serum
Secondary ID 
A test code used for billing and in test definitions created prior to November 2011
NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.
Useful For Suggests clinical disorders or settings where the test may be helpful
Monitoring treatment with synthetic hormones (synthetic triiodothyronine will cause a low total thyroxine [T4]).
Monitoring treatment of hyperthyroidism with thiouracil and other antithyroid drugs.
Total T4 levels offer a good index of thyroid function when the thyroid-binding globulin is normal and nonthyroidal illness is not present.
Testing Algorithm Delineates situation(s) when tests are added to the initial order. This includes reflex and additional tests.
See Thyroid Function Ordering Algorithm in Special Instructions.
Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test
Method Name A short description of the method used to perform the test
Immunoenzymatic Assay
Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name
Aliases Lists additional common names for a test, as an aid in searching
T4 Total Only, S MMLNE only
T4, Serum
Tetraiodothyronine
Thyroid Hormone
Thyroxine (ORDER 8724 if not specified)
Thyroxine (T4), Total & Free ORDER 8684
Thyroxine, Total
Specimen Type Describes the specimen type needed for testing
Specimen Required Defines the optimal specimen. This field describes the type of specimen required to perform the test and the preferred volume to complete testing. The volume allows automated processing, fastest throughput and, when indicated, repeat or reflex testing.
Container/Tube:
Preferred: Red top
Acceptable: Serum gel
Specimen Volume: 0.6 mL
Specimen Minimum Volume Defines the amount of specimen required to perform an assay once, including instrument and container dead space. Submitting the minimum specimen volume makes it impossible to repeat the test or perform confirmatory or perform reflex testing. In some situations, a minimum specimen volume may result in a QNS (quantity not sufficient) result, requiring a second specimen to be collected.
Reject Due To Identifies specimen types and conditions that may cause the specimen to be rejected
| Hemolysis | Mild OK; Gross reject |
| Lipemia | Mild OK; Gross OK |
| Icterus | NA |
| Other | NA |
Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.
| Specimen Type | Temperature | Time |
|---|---|---|
| Serum | Refrigerated (preferred) | 7 days |
| Frozen | 90 days |
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Thyroxine (T4) is a metabolically active thyroid hormone containing 4 atoms of iodine (tetraiodothyronine; T4). 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 triidothyronine (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).
See Thyroid Function Ordering Algorithm in Special Instructions.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
> or =12 months: 5.0-12.5 mcg/dL
Reference values have not been established for patients that are <12 months of age.
Interpretation Provides information to assist in interpretation of the test results
Values >12.5 mcg/dL are seen in hyperthyroidism and patients with acute thyroiditis.
The normal concentration for newborns is much higher than for adults. Values up to 23 mcg/dL are generally signified as normal in the literature and values <12 mcg/dL for a newborn may signify hypothyroidism.
Values <5.0 mcg/dL 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 patients with chronic liver disease, or nephrosis (decreased TBG levels).
A TRH 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 Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
In pregnancy, total T4 might be falsely-low with this assay and should not be used as the only marker for thyroid disease 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.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
Wilson JD, Foster DW, Kronenburg MD, et al: Williams textbook of Endocrinology. 9th edition, WB Saunders Company, 1998
Method Description Describes how the test is performed and provides a method-specific reference
The instrument used is a Beckman Coulter DXI 800. The Access Total thyroxine (T4) assay is competitive-binding immunoenzymatic assay. A sample is added to a reaction vessel with antithyroxine antibody, thyroxine-alkaline phosphatase conjugate, and para-magnetic particles coated with goat antimouse capture antibody and a stripping agent to dissociate all T4 from binding proteins. Thyroxine in the sample competes with the thyroxine-alkaline phosphatase conjugate for binding sites on a limited amount of specific antithyroxine antibody. Resulting antigen: antibody complexes bind to the capture antibody on the solid phase. After incubation in a reaction vessel, materials bound to the solid phase are held in a magnetic field, while unbound materials are washed away. The chemiluminescent substrate Lumi-Phos 530 is added to the vessel and light generated by the reaction is measured with a luminometer. The light production is inversely proportional to the concentration of thyroxine in the sample. The amount of analyte in the sample is determined from a stored, multipoint calibration curve. (Assay Manual, Beckman Coulter Inc, Fullerton, CA 2009)
Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.
Monday through Friday 5 a.m. – 12 a.m., Saturday 6 a.m. – 6 p.m.
Analytic Time Defines the amount of time it takes the laboratory to setup and perform the test. This is defined in number of days. The shortest interval of time expressed is "same day/1 day," which means the results may be available the same day that the sample is received in the testing laboratory. One day means results are available 1 day after the sample is received in the laboratory.
Maximum Laboratory Time Defines the maximum time from specimen receipt at Mayo Medical Laboratories until the release of the test result
Specimen Retention Time Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded
Performing Laboratory Location The location of the laboratory that performs the test
Test Classification Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer's instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR), Investigation Use Only (IUO) product, or a Research Use Only (RUO) product.
CPT Code Information Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Medical Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.
84436
LOINC® Code Information Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the result codes returned for this test or profile.
| Result ID | Reporting Name | LOINC Code |
|---|---|---|
| T4 | T4 (Thyroxine), Total Only, S | 3026-2 |
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