Sign in →

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

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.

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

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.

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.

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

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.

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.

Clinical Reference

Wilson JD, Foster DW, Kronenburg MD, et al: In Williams Textbook of Endocrinology. Ninth edition. WB Saunders Company, 1998

Analytic Time

Same day/1 day

Reject Due To

Hemolysis

Mild OK; Gross reject

Lipemia

Mild OK; Gross OK

Icterus

Mild OK; Gross OK

Other

NA

Method Name

Electrochemiluminescence Immunoassay