T4, Free

T4, Free

Alternate Test Name

Free Thyroxine

Epic Mnemonic
Sunquest Mnemonic

LAB127
FT4

Category

Chemistry

Test Performance Schedule

Sunday - Saturday

Result Availability

Within 24 hours

Container

Gold top (SST), Red top (serum), or Green top (lithium heparin) tube

Volume

Pref. Vol.: 1.0 mL serum or plasma
Min. Vol.: 0.5 mL serum or plasma

Collection Instructions

• Routine venipuncture

• Immediately after collection, gently invert tube 5-10 times

• Clot 30 minutes

• Promptly centrifuge 10 minutes

• If no gel barrier is present immediately transfer serum or plasma to a plastic tube and refrigerate

• Properly centrifuged gel barrier tube does not require transfer of serum or plasma to separate tube

Transportation Instructions

Refrigerated

Stability

• Refrigerated: 5 days

Remarks

Medicare Medical Necessity Policy

Biotin, also referred to as Vitamin B7 or Vitamin H, can interfere with this test when taken in mega doses of 5mg (5000 mcg) or more. It is often promoted as a skin and hair beauty aid. Please ask your patients to refrain from taking Biotin or supplements containing Biotin for at least 24 hours before collecting specimens for lab testing.

https://biotinfacts.roche.com/

https://www.fda.gov/medicaldevices/safety/alertsandnotices/ucm586505.htm

CPT Codes

84439

Effective/Revised

7/30/2019

Thyroxine (T4) is the main thyroid hormone secreted into the bloodstream by the thyroid gland. Together with triiodothyronine (T3) it plays a vital role in regulating the body’s metabolic rate, influences the cardiovascular system, growth and bone metabolism, and is important for normal development of gonadal functions and nervous system.

T4 circulates in the bloodstream as an equilibrium mixture of free and serum bound hormone. Free T4 (FT4) is the unbound and biologically active form, which represents only 0.03 % of the total T4. The remaining T4 is inactive and bound to serum proteins such as thyroxine binding globulin (75 %), pre-albumin (15 %), and albumin (10 %).

The determination of free T4 has the advantage of being independent of changes in the concentrations and binding properties of the binding proteins; additional determination of a binding parameter (T-uptake, TBG) is therefore unnecessary.

Therefore free T4 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status. It should be measured together with TSH if thyroid disorders are suspected and is also suitable for monitoring thyrosuppressive therapy.