Test Code 1016
Test Details
Free T4 (Free Thyroxine or FT4)
Free T4 is the amount of T4 hormone that is freely circulating in the blood.
Methodology
Chemiluminescence immunoassay
Patient Preparation
None
Preferred Specimen
1.0 mL serum collected in a serum separator tube (SST / Tiger Top)
Alternate Specimen
1.0 mL plasma in an EDTA plasma separator tube (Pearl Top)
Transport Temperature
Refrigerated (ship on frozen cold packs)
Stability
Refrigerated: 7 days (7 days on SST gel)
Lab Values
Free T4 (Free Thyroxine or FT4)
- Low: <0.93 ng/dL
- Optimal: 0.93-1.70 ng/dL
- High: >1.70 ng/dL
Clinical Significance
Free T4 (Free Thyroxine or FT4)
- Low Free T4 levels are consistent with an underactive thyroid gland or inadequate thyroid hormone replacement.1-3
- High Free T4 levels are consistent with either an overactive thyroid gland or excess thyroid hormone replacement.1-3
- High doses of biotin (> 5 mg/day) may interfere with assay results. Patient assumed to be refraining from biotin supplementation for at least 3 days prior to blood draw.
References:
- Goodman N, Cobin RH. Reproductive disorders. In: Camacho PM, Gharib H, Sizemore GW, eds. Evidence-Based Endocrinology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins. 2012:190-228.
- Castro MR, Gharib H. Thyroid disorders. In: Camacho PM, Gharib H, Sizemore GW, eds. Evidence-Based Endocrinology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins. 2012:43-80.
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012;18(6):988-1028.
Treatment Options
Free T4 (Free Thyroxine or FT4)
Treatment is based on the cause of elevation/reduction.
