THYROXINE, FREE (FREE T4) - (P)
Test Code: FT4
Free T4 is available as a component of the Thyroid Reflex Testing protocol.
Free T4 will be performed when TSH is < 0.40 mU/L or ≥ 4.3 mU/L
Test Indications: This test is used in the evaluation of suspected thyroid dysfunction when measured in conjunction with thyroid stimulating hormone. Free T4 is the unbound and biologically active form & represents only 0.03% of total circulating thyroxine.
Patient Preparation Instructions: Multivitamins (45–125 µg biotin) or biotin-only supplements up to 1 mg per day do not interfere with this test. Samples should not be taken from patients receiving therapy with high biotin doses (i.e. >5 mg/day) until at least 8 hours after the last dose.
For additional information see:
Plasma: 2.0 mL
Stability 7 days refrigerated, 1 month frozen
Serum: 2.0 mL
Samples must be transferred to an aliquot tube and stored frozen if analysis will not be complete within 48 hours (#110-10-05 Serum / Plasma Separation Procedure & Transport)
Free T4 is automatically performed when TSH < 0.40 mU/L and ≥ 4.3 mU/L except for ENDO or ENDM. Separate plasma and refrigerate.