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Test Code FT4 T4 (Thyroxine), Free, Plasma or Serum

Important Note

Many dietary supplements promoted for hair, skin, and nail benefits contain Biotin (Vitamin B7) levels up to 650 times the recommended daily intake.  High levels of Biotin can cause clinically significant incorrect lab test results, resulting in falsely high or falsely low results, depending on the test. Patients taking high doses of Biotin should be instructed to discontinue Biotin for at least 12 hours prior to lab testing.

Performing Laboratory

MaineGeneral Medical Center

Reference Values

Adults: 0.76-1.46 ng/dL

Critical value (automatic call-back):  >4.0 ng/mL

Day(s) Test Performed

Monday through Sunday

Specimen Requirements

Submit only 1 of the following specimens:

Preferred:

Plasma

Draw blood in a green-top (lithium heparin) tube(s). For pediatric specimen, draw blood in a green-top (lithium heparin) MICROTAINER(S)®. Spin down and send 1 mL of lithium heparin plasma refrigerated. Label specimen with patient’s name (first and last), date of birth, date and time of draw, phlebotomist’s initials, and as plasma.

Alternate:

Serum

Draw blood in a gold-top (serum separater) gel tube(s). Spin down and send 1 mL of serum refrigerated. Label specimen with patient’s name (first and last), date of birth, date and time of draw, phlebotomist’s initials, and as serum.

 

Moderately hemolyzed specimens are not acceptable.

 

Specimen stability:

8 hours at room temperature

2 days refrigerated at 2-8°C

Indefinitely when frozen at  ≤ -20°C

 

Minimum volume:    1 mL
 

Test Classification and CPT Coding

84439

Methodology

Chemiluminescent immunoassay