Triiodothyronine (free) - serum

Last updated: Wednesday, 31, March, 2004

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Item Process
Specimen

5 mL blood in plain tube.

Method

Radioimmunoassay.

Reference Interval

4.0-8.0 pmol/L.

Application

Thyroid stimulating hormone is the preferred initial test in the assessment of thyroid function.

The primary indication for free T3 is for the diagnosis of T3 toxicosis (that is, suppressed TSH and normal free T4). Although occasionally used as an early or confirmatory test for thyrotoxicosis (free T3 is often elevated before free T4), it is not required to establish this diagnosis.

It is an unreliable test for hypothyroidism, and should not be used.

Occasionally, the result may be suspected, on clinical grounds, of being anomalous; total T3 may assist to clarify the situation.

Interpretation

Increased levels of free T3 are found in T3 toxicosis and in thyrotoxicosis. Free T3 may be elevated earlier than free T4 in thyrotoxicosis.

Reference

Braverman LE and Utiger RD eds. Werner and Ingbar’s The Thyroid. A Fundamental and Clinical Text. 7th ed. Harper and Row 1996.