Sign in →

Test ID: T4FT4 T4 (Thyroxine), Total and Free, Serum

Reporting Name

T4 (Thyroxine), Total and Free

Useful For

Assessment of thyroid function when thyroid function disorders are suspected

Clinical Information

Total Thyroxine:

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), 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. However, changes in binding proteins can occur which affect the level of total T4 but leave the level of unbound hormone unchanged.

 

Free Thyroxine:

Free thyroxine comprises a small fraction of total thyroxine. The free T4 (FT4) is available to the tissues and is, therefore, the metabolically active fraction.

 

Elevations in FT4 cause hyperthyroidism, while decreases cause hypothyroidism.

Profile Information

Test ID Reporting Name Available Separately Always Performed
T4F T4 (Thyroxine), Free, S No Yes
T4CC T4 (Thyroxine), Total Only, S No Yes

Report Available

Same day/1 to 2 days

Day(s) Performed

Monday through Sunday

Clinical Reference

Melmed S, Polonsky KS, Larsen PR, et al: Williams Textbook of Endocrinology. 12th ed. Elsevier Saunders; 2011:348-414

Method Name

Electrochemiluminescence Immunoassay (ECLIA)

Specimen Type

Serum


Specimen Required


Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Specimen Volume: 1 mL


Specimen Minimum Volume

0.625 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 7 days
  Frozen  30 days
  Ambient  72 hours

Reference Values

T4 (THYROXINE), TOTAL ONLY

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

 ≥20 years: 4.5-11.7 mcg/dL

 

 

T4 (THYROXINE), FREE

0-5 days: 0.9-2.5 ng/dL

6 days-2 months: 0.9-2.2 ng/dL

3-11 months: 0.9-2.0 ng/dL

1-5 years: 1.0-1.8 ng/dL

6-10 years: 1.0-1.7 ng/dL

11-19 years: 1.0-1.6 ng/dL

 ≥20 years: 0.9-1.7 ng/dL

Test Classification

This test has been cleared, approved, or is exempt by the US 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-Total

84439-Free

LOINC Code Information

Test ID Test Order Name Order LOINC Value
T4FT4 T4 (Thyroxine), Total and Free 90224-7

 

Result ID Test Result Name Result LOINC Value
T4F T4 (Thyroxine), Free, S 83122-2
T4CC T4 (Thyroxine), Total Only, S 83119-8
Mayo Clinic Laboratories | Endocrinology Catalog Additional Information:

mml-endo-thyroid