References1-7 Triiodothyronine (T3) is the hormone principally responsible for the development of the effects of the thyroid hormones on the various target organs. T3 (3,5,3’- triiodothyronine) is mainly formed extrathyroidally, particularly in the liver, by enzymatic 5’-deiodination of T4. Accordingly, the T3 concentration in serum is more a reflection of the functional state of the peripheral tissue than the secretory performance of the thyroid gland. A reduction in the conversion of T4 to T3 results in a decrease in the T3 concentration. It occurs under the influence of medicaments such as propanolol, glucocorticoids or amiodarone and in severe non-thyroidal illness (NTI), and is referred to as “low T3 syndrome”. As with T4, over 99 % of T3 is bound to transport proteins. However, the affinity of T3 to them is around 10-fold lower. The determination of T3 is utilized in the diagnosis of T3-hyperthyroidism, the detection of early stages of hyperthyroidism and for indicating a diagnosis of thyrotoxicosis factitia.