Thyroxine (T4)
Also known as: T4, Total, Tetraiodothyronine
Use
Thyroid function test. Decreased in hypothyroidism and in the third stage of (painful) subacute thyroiditis; increased with hyperthyroidism, with subacute thyroiditis in its first stage and with thyrotoxicosis due to Hashimoto disease.3 Used to diagnose T4 toxicosis.
Special Instructions
Patients should be asked about biotin supplementation, as high doses can interfere with the test. It is recommended to cease biotin consumption at least 72 hours prior to sample collection. The combination of serum T4 and T3 uptake is used to assess serum TBG levels, helping to identify the cause of abnormal T4 readings.
Limitations
T4 levels may be abnormally altered due to excess iodine intake or surreptitious use of thyroxine. Systemic nonthyroidal diseases can affect T4 levels. Changes in TBG binding capacity or quantity can lead to elevated or decreased T4 without corresponding symptoms. Elevated T4 in nonthyroidal diseases might be due to liver disease. Familial dysalbuminemic hyperthyroxinemia, a euthyroid syndrome, may cause an increase in serum thyroxine and the free thyroxine index (FTI), sometimes mistaken for thyroid dysfunction.
Methodology
Immunoassay (ECLIA)
Biomarkers
LOINC Codes
- 3026-2
- 3026-2
Result Turnaround Time
1 day
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
1 mL (adult), 0.8 mL (pediatric)
Minimum Volume
0.7 mL (adult), 0.3 mL (pediatric) (Note: This volume does not allow for repeat testing.)
Container
Red-top tube, gel-barrier tube or green-top (lithium heparin) tube; do not use oxalate, EDTA or citrate plasma
Collection Instructions
If a red-top tube or plasma is used, transfer separated serum or plasma to a plastic transport tube.
Patient Preparation
Patients should stop biotin consumption at least 72 hours before collection.
Storage Instructions
Room temperature
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 14 days |
| Refrigerated | 14 days |
| Frozen | 14 days |