• 5-diiodo-L-thyronine
• thyroid hormone
• endogenous detection

The processes and pathways mediating the intermediary metabolism of carbohydrates, lipids, and proteins are all affected by thyroid hormones (THs) in almost all tissues. Although many studies have reported the effect of thyroxine (T4) and triiodo-L-thyronine (T3) on metabolism, only a few studies have investigated the biological actions of 3,5-diiodo-L-thyronine (T2). Until recent years, T2, because of its low affinity for thyroid hormone receptors (THR), was considered an inactive metabolite of T4 and T3. Several observations, however, led to a reconsideration of this idea. Early studies dealing with the biological activities of this iodothyronine revealed its ability to stimulate cellular/ mitochondrial respiration by a nuclear-independent pathway. Mitochondria and bioenergetic mechanisms seem to be major targets of T2, although outside the mitochondria T2 also has effects on carriers, ion-exchangers, and enzymes. Recent studies suggest that T2 may also affect the transcription of some genes, but again the underlying mechanisms seem to be different from those actuated by T3.

Early published data obtained using Antibody-based immunoassay gave controversial results concerning endogenous T2 concentration, and another sensitive assay to further confirm these data has not been yet developed. For this reason, we have developed an offline pre-analytical sample preparation (sample extraction) for all THs from human serum and a High Performance Liquid Chromatography- tandem Mass Spectrometry (HPLC-MS/MS) method for quantification of such analytes. Additionally, the thesis project was also integrated with assessing the effect of T2 on glucose metabolism and T2 uptake in hepatocellular carcinoma (HepG2) and rat cardiomyoblast (H9c2) cell lines. We also studied cardiac effects of T2 in an isolated rat heart model.