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Unit Code 9404:
T3 (Triiodothyronine), Free, Serum

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Useful For

Free T3 is a second- or third-level test of thyroid function. It provides

further confirmation of hyperthyroidism, supplementing the T4,

sensitive thyrotropin (sTSH), and total T3 assays.

 

Evaluating clinically euthyroid patients who have an altered

distribution of binding proteins

 

Monitoring thyroid hormone replacement therapy

Clinical Information

Normally T3 circulates tightly bound to thyroxine-binding globulin

and albumin. Only 0.3% of the total T3 is unbound, but the free

fraction is the active form.

 

In hyperthyroidism, both thyroxine (tetraiodothyronine;T4) and T3

levels (total and free) are usually elevated, but in a small subset of

hyperthyroid patients (T3 toxicosis) only T3 is elevated. Generally,

free T3 (FT3) measurement is not necessary since total T3 will suffice.

However, free T3 levels may be required to evaluate clinically

euthyroid patients who have an altered distribution of binding proteins

(eg, pregnancy, dysalbuminemia).

 

Some investigators recommend the free T3 assay for monitoring

thyroid replacement therapy, although its clinical role is not precisely

defined.

Reference Values

> or =1 year: 2.0-3.5 pg/mL

 

(U for NIH)

> or =1 year: 200-350 pg/dL

Interpretation

Elevated FT3 values are associated with thyrotoxicosis or excess

thyroid hormone replacement.

Cautions

FT3 is not a sensitive test for hypothyroidism.

 

Some patients who have been exposed to animal antigens, either

in the environment or as part of treatment or imaging procedures,

may have circulating antianimal antibodies present. These anti-

bodies may interfere with the assay reagents to produce unreliable

results.

Clinical Reference

1.   Hay ID, Klee GG:  Thyroid dysfunction.  Endocrinol Metab Clin

      North Am 1988;17:473-509

 

2.   Wilson JD, Foster DW, Kronenburg MD, Larsen PR:  Williams

      Textbook of Endocrinology. 9th edition. WB Saunders Company,

      1998

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