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Free T4 (Free Thyroxine or FT4)
Free T4 is the amount of T4 hormone that is freely circulating in the blood.
1.0 mL serum collected in a serum separator tube (SST / Tiger Top)
1.0 mL plasma in an EDTA plasma separator tube (Pearl Top)
Refrigerated (ship on frozen cold packs)
Refrigerated: 7 days (7 days on SST gel)
- Low: <0.93 ng/dL
- Optimal: 0.93-1.70 ng/dL
- High: >1.70 ng/dL
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.