Special Precautions/Comments:

Please indicate on request form if patient is on any thyroid treatment. 

Additional Information:

T3 (triiodothyronine) is a thyroid hormone secreted in response to thyroid stimulating hormone (TSH). T3 is also made in the periphery by deiodination of T4 (thyroxine). The majority (>99%) of T3 circulates bound to transporter proteins such as thyroxine binding globulin (TBG) and albumin. The remaining T3 is free in the blood and biologically available to carry out its metabolic function.  

Free T3 is therefore measured to assess thyroid function in the investigation of hypothyroidism and hyperthyroidism. Free T3 may be added to a thyroid function test if the TSH is outside the reference range and the free T4 is unexpected, such as in subclinical (‘borderline’) cases where the free T4 may be normal. For example, in ‘T3 toxicosis’, the free T4 is normal but the free T3 is elevated. Non-thyroidal illness can cause transiently abnormal thyroid function tests. Free T3 is also measured in patients on amiodarone because it inhibits the deiodination of T4, resulting in decreased production of T3.