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TEST ID T4 T4 (Thyroxine), Total Only, Serum

Reporting Name

T4 (Thyroxine), Total Only, S

Specimen Type

Serum


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


Patient Preparation: For 4 to 6 weeks before specimen collection, patient should not be taking lipid-lowering medications containing dextrothyroxine.

Supplies: Sarstedt Aliquot Tube, 5 mL (T914)

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 Minimum Volume

0.75 mL

Specimen Stability Information

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

Testing Algorithm

For information see Thyroid Function Ordering Algorithm.

Method Name

Electrochemiluminescence Immunoassay

Reject Due To

Gross hemolysis Reject
Gross lipemia OK
Gross icterus OK

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 International System of Units (SI) conversion for Reference Values, see  www.mayocliniclabs.com/order-tests/si-unit-conversion.html

Day(s) Performed

Monday through Saturday

Report Available

1 to 3 days

Specimen Retention Time

2 weeks

Performing Laboratory

Mayo Clinic Laboratories in Rochester

CPT Code Information

84436

Forms

If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen. 

Useful For

Monitoring treatment with synthetic hormones (synthetic T3 [triiodothyronine] will cause a low total T4 [thyroxine])

 

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

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 (previously 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.