Laboratory: Clinical Biochemistry
Test Name:
THYROXINE, FREE (FREE T4) - (P)
Test Code: FT4
Clinical Information:
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: 
Collection Devices:
Specimen Required:


Plasma: 2.0 mL

Stability 7 days refrigerated, 1 month frozen
Referral:
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)
Requisition:
Reference Values:
0 - 7 d: 20.8-54.4   pmol/L                           
8 d - 4 wk:   9.6-24.0
5 wk - 12 mo:   8.0-18.4
13 mo - 5 y:   8.0-21.6
6 - 10 y: 10.4-21.6
11 - 20 y:   8.0-20.8
≥ 21 y:   9.7-25.7
Availability:
Daily
See Also:
More Information:
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.