Free triiodothyronine (free T3)

What it measures

Free T3 is the unbound form of the most biologically active thyroid hormone. It is mainly used when TSH is suppressed but free T4 is normal, to detect T3 toxicosis.

Specimen and preparation

• Specimen: Serum or lithium-heparin plasma
• Tube: Gold top

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Graves disease and toxic nodular goitre
• T3 toxicosis
• Liothyronine therapy

Causes of low values

• Non-thyroidal illness (sick euthyroid)
• Starvation
• Hypothyroidism (late fall)
• Amiodarone, beta-blockers, steroids

Method

Chemiluminescent immunoassay

Good to know

Of limited use for diagnosing hypothyroidism, as it falls late.

Related tests