T4, Free
Test Mnemonic
FT4
CPT Codes
- 84439 - QTY (1)
LOINC ®
3024-7
Aliases
- Thyroxine Free
Performing Laboratory
Cleveland Clinic Laboratories
Specimen Requirements
| Volume | Type | Container | Collect Temperature | Transport Temperature | Special Instructions |
|---|---|---|---|---|---|
| 1 mL | Plasma | Lithium heparin PST (Lt. Green) | Centrifuge and refrigerate. | Submit in original tube or aliquot into CCL aliquot tube |
Alternate Specimen Requirements
| Volume | Type | Container | Collect Temperature | Transport Temperature | Special Instructions |
|---|---|---|---|---|---|
| 1 mL | Serum | SST (Gold) | Centrifuge and refrigerate. |
Minimum Specimen Requirements
| Volume | Type | Container | Collect Temperature | Transport Temperature | Special Instructions |
|---|---|---|---|---|---|
| 0.5 mL |
Stability
| Environmental Condition | Description |
|---|---|
| Ambient | 5 days |
| Refrigerated | 7 days |
| Frozen | 30 days |
Days Performed
Sun - Sat
Turnaround Time
8 hours
Methodology
| Name | Description |
|---|---|
| Electro Chemiluminescence Immunoassay (ECLIA) |
Special Info
Clinical Info
Thyroxine (T4) is one of the major thyroid hormones, along with triiodothyronine (T3). T4 is the most abundant thyroid hormone, and it regulates metabolism, growth, and development. Free T4 is the active form of thyroxine, and it is generally thought to be more accurate than total T4 for testing for thyroid hormone function because free T4 is not affected by protein levels. Free T4 represents just a fraction of total circulating thyroxine, ranging from 0.03% to 0.05%. Testing for free T4 is commonly ordered in response to an abnormal thyroid-stimulating hormone (TSH) result. An elevated free T4 result is consistent with an overactive thyroid (hyperthyroidism), and a decreased free T4 result is consistent with an underactive thyroid (hypothyroidism). Free T4 may also be used to evaluate thyroid function in patients with alternations in thyroxine binding globulin (TBG).