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T4 Free

Free T4; Thyroxine, Free

Tube Type = Mint Green Top

Tube Type

Test ID
LAB127
EPIC Order
T4, FREE
CPT Code(s)
84439
Group/Individual Test
Individual
Laboratory
Core Laboratory
Tube Station
30; STAT: 888
Specimen
Routine:
4 mL blood/green lithium heparin gel tube. Also acceptable gold SST tube or red plain tube
Micro:
0.9 mL blood/mint green top bullet/gold top/red top
Stability
Unspun: 8 hours
Spun/separated, Room Temperature-<8 hrs recommended
Spun/separated, Refrigerator: 48 hours
Spun/separated, Frozen: 30 days
Availability
Routine:
24 hrs/day
STAT:
Yes
Turnaround Time
Routine: 2 hrs; STAT: 1 hr
Reference Range
Normal:
T4 Free (ng/dL)
Age Range
<1 month Not Specified
1 month - 2 years 0.94-1.44
2 - 12 years 0.86-1.40
12 - 20 years 0.83-1.43
>20 years 0.89-1.76
Reference Ranges effective 7/13/2021
Comments
  • The anticonvulsant drug phenytoin may interfere with total and free T4 levels due to competition for TBG binding sites.
  • Results from patients taking biotin supplements should be interpreted with caution due to possible interference.
  • Thyroid autoantibodies in human serum may interfere and cause falsely elevated free T4 results.
  • Free T4 values may be decreased in patients with non-thyroidal conditions and in patients taking carbamazepine.

Reviewed by J. Carr on January 02, 2026

UNC Hospitals
McLendon Clinical Laboratories
101 Manning Drive
Chapel Hill, NC 27514