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Unit Code 200050:
T4 (Thyroxine), Free, Serum

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Useful For

The FT4 value combined with the TSH value gives a more accurate

picture of the thyroid status in patients with abnormal TBG levels such as

those who are pregnant or those who are receiving treatment with

estrogens, androgens, dilantin, or salicylates.

Clinical Information

Free thyroxine (FT4) comprises a small fraction of total T4. The free

T4 is available to the tissues and is, therefore, the metabolically
active fraction.

 

Elevations in FT4 result in hyperthyroidism, while decreases result

in hypothyroidism.

 

The test generally is used as a second line test after thyroid-

stimulating hormone (TSH) to help evaluate TSH changes.

 

FT4 works well to correct total T4 values for thyroxine-binding

globulin (TBG) alterations, but may give misleading values when

abnormal binding proteins are present or the patient has other

major illnesses ("Euthyroid Sick Syndrome").

Reference Values

0.8-1.8 ng/dL

Interpretation

Elevated values suggest hyperthyroidism or exogenous thyroxine.

Decreased values suggest hypothyroidism.

Cautions

Heparin, free fatty acids, and autoantibodies to T4 can cause

misleading values. Phenytoin may interfere with FT4 levels due to

competition with T4 for TBG binding sites.

Special Instructions and Forms

• Thyroid Function Algorithm 

Clinical Reference

Klee GG, Hay ID: Biochemical thyroid function testing. Mayo Clin Proc

69:469-470, 1994

Key