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HelpEpic Test Code LAB126 T4 (Thyroxine), Total Only, Serum
Additional Codes
MML Code: T4
LIS Code: T4T
NY State Approved
YesPerforming Laboratory
Mayo Clinic Laboratories in Rochester
Reporting Name
T4 (Thyroxine), Total Only, SMethod Name
Electrochemiluminescence Immunoassay
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 7 days | |
| Frozen | 30 days |
Ordering Guidance
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.
Specimen Required
Collection Container/Tube:
Preferred: Serum gel
Acceptable: Red top
Submission Container/Tube: Plastic vial
Specimen Volume: 1 mL
Collection Instructions: Centrifuge and aliquot serum into a plastic vial.
Specimen Type
SerumSpecimen Minimum Volume
0.75 mL
Reference Values
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
Adult (≥20 years): 4.5-11.7 mcg/dL
For SI unit Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html
Report Available
1 to 3 daysDay(s) Performed
Monday through Friday
CPT Code Information
84436
Reject Due To
| Gross hemolysis | Reject |
| Gross lipemia | OK |
| Gross icterus | OK |
Useful For
Monitoring treatment with synthetic hormones (synthetic triiodothyronine [T3] will cause a low total thyroxine [T4])
Monitoring treatment of hyperthyroidism with thiouracil and other anti-thyroid drugs
Index of thyroid function when the thyroxine-binding globulin is normal and non-thyroidal illness is not present
Testing Algorithm
For information see Thyroid Function Ordering Algorithm.
Special Instructions
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, ie, 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 and non-thyroidal illness. However, changes in binding proteins can occur that affect the level of total T4 but leave the level of unbound hormone unchanged.
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 in 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 due to 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 thyrotropin, 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.
Thyrotropin (TSH) may be better than 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.
In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA]or heterophile antibodies), which may cause interference in some immunoassays. The presence of antibodies to streptavidin or ruthenium also rarely occur and may interfere with this assay. Caution should be used in interpretation of results, and the laboratory should be alerted if the result does not correlate with the clinical presentation.
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.
Serum biotin concentrations up to 1200 ng/mL do not interfere with this assay. Concentrations up to 1200 ng/mL may be present in specimens collected from patients taking extremely high doses of biotin up to 300 mg per day.(1) In a study among 54 healthy volunteers, supplementation with 20 mg/day biotin resulted in a maximum serum biotin concentration of 355 ng/mL 1-hour post-dose.(2)