Imperial Collage Healthcare

Free T3 (free triiodothyronine)


Test Background
T3 is more potent than T4 and both circulate in the blood as an equilibrium mixture of free and protein-bound hormone. T3 is bound to thyroxine binding globulin (TBG) and albumin. The free fraction, 0.2-0.4 % of the total T3, is present in solution as unbound or free T3 (FT3) and represents the physiologically active thyroid hormone. FT3 is typically elevated to a greater degree than free thyroxine (FT4) in Graves’ disease and in toxic adenomas. Free T3 alone is elevated (T3 thyrotoxicosis) in about 5 % of the hyperthyroid population.


Clinical Indications

  • Diagnosis and management of thyrotoxicosis
  • Differential diagnosis of hyperthyroid disorders
  • Management of patients on amiodarone


Reference Range

2.5-5.7 pmol/L


Sample Required
SST (gold top)


Sample Volume
0.5 mL


Turnaround Time
1 day


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