An online solid-phase extraction–liquid chromatography–tandem mass spectrometry method to study the presence of thyronamines in plasma and tissue and their putative conversion from 13C6-thyroxine

Thyronamines are exciting new players at the crossroads of thyroidology and metabolism. Here, we report the development of a method to measure 3-iodothyronamine (T1AM) and thyronamine (T0AM) in plasma and tissue samples. The detection limit of the method was 0.25 nmol/l in plasma and 0.30 pmol/g in tissue both for T1AM and for T0AM. Using this method, we were able to demonstrate T1AM and T0AM in plasma and liver from rats treated with synthetic thyronamines. Although we demonstrated the in vivo conversion of 13C6-thyroxine (13C6-T4) to 13C6-3,5,3′-triiodothyronine, we did not detect 13C6-T1AM in plasma or brain samples of rats treated with 13C6-T4. Surprisingly, our method did not detect any endogenous T1AM or T0AM in plasma from vehicle-treated rats, nor in human plasma or thyroid tissue. Although we are cautious to draw general conclusions from these negative findings and in spite of the fact that insufficient sensitivity of the method related to extractability and stability of T0AM cannot be completely excluded at this point, our findings raise questions on the biosynthetic pathways and concentrations of endogenous T1AM and T0AM.