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HelpTest ID: T4FT4 T4 (Thyroxine), Total and Free, Serum
Reporting Name
T4 (Thyroxine), Total and FreeUseful For
Thyroxine (T4) and free T4 are measured together with thyroid-stimulating hormone when thyroid function disorders are suspected.
Clinical Information
THYROXINE (T4), TOTAL:
T4 is synthesized in the thyroid gland. T4 is metabolized to 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 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).
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.
THYROXINE (T4), FREE:
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 |
Analytic Time
Same day/1 dayDay(s) and Time(s) Performed
Monday through Sunday; Continuously
Clinical Reference
Melmed S, Polonsky KS, Larsen PR, et al: Williams textbook of Endocrinology. 12th edition. Elsevier Saunders Company, 2011, pp 348-414
Method Name
Electrochemiluminescence Immunoassay
Specimen Type
SerumSpecimen 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
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
T4 (THYROXINE), FREE
Adult (≥20 years of age): 0.9-1.7 ng/dL
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
Test Classification
This test has been cleared, approved or is exempt 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-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 |
mml-endo-thyroid