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HelpTest ID: T4FT4 T4 (Thyroxine), Total and Free, Serum
Reporting Name
T4 (Thyroxine), Total and FreeClinical 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, 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. However, changes in binding proteins can occur that 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 daysDay(s) Performed
Monday through Sunday
Clinical Reference
1. Peyro Saint Paul L, Debruyne D, Bernard D, Mock DM, Defer GL: Pharmacokinetics and pharmacodynamics of MD1003 (high-dose biotin) in the treatment of progressive multiple sclerosis. Expert Opin Drug Metab Toxicol. 2016;12(3):327-344
2. Grimsey P, Frey N, Bendig G, et al: Population pharmacokinetics of exogenous biotin and the relationship between biotin serum levels and in vitro immunoassay interference. J Pharmacokinet Pharmacodyn. 2017 Sept;2(4):247-256. doi: 10.4155/ipk-2017-0013
3. Melmed S, Polonsky KS, Larsen PR, et al: Williams Textbook of Endocrinology. 12th ed. Elsevier Saunders; 2011:348-414
Method Name
Electrochemiluminescence Immunoassay
Specimen Type
SerumSpecimen 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 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
84439
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