T4
Test Mnemonic
T4
CPT Codes
- 84436 - QTY (1)
LOINC ®
3026-2
Aliases
- Thyroxine
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 | 4 days |
| Refrigerated | 8 days |
| Frozen | 12 months |
Days Performed
Sun - Sat
Turnaround Time
8 hours
Methodology
| Name | Description |
|---|---|
| Electro Chemiluminescence Immunoassay (ECLIA) |
Special Info
Patients taking a biotin dose of up to 5 mg/day should refrain from taking biotin for 4 hours prior to sample collection. Patients taking a biotin dose of 5 to 10 mg/day should refrain from taking biotin for 8 hours prior to sample collection. Patients taking a biotin dose > 10 mg/day should consult with their physician or the laboratory prior to having a sample taken. Clinicians should consider biotin interference as a source of error, when clinically suspicious of the laboratory result.
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. This assay is used as a measure of total thyroid secretion of T4, both bound and free. Because almost all of the circulating thyroxine is protein-bound (>99%), testing for total T4 can vary based on changes in serum thyroid hormone-binding proteins such as thyroxine binding globulin (TBG), transthyretin (TTR), or albumin. Due to this potential variability, testing for free T4 is generally a better indicator of thyroid function than total T4. An elevated total T4 result is consistent with an overactive thyroid (hyperthyroidism), and a decreased total T4 result is consistent with an underactive thyroid (hypothyroidism).