Identification and quantification of 3-iodothyronamine metabolites in mouse serum using liquid chromatography–tandem mass spectrometry

3-Iodothyronamine (T1AM) is an endogenous derivative of thyroxine. Recently there have been numerous reports of analytical methods to quantify endogenous T1AM levels, but substantial discrepancies in concentration depending on the method of analysis (LC–MS/MS or immunoassay) suggest endogenous T1AM may be covalently modified in vivo. Using information dependent acquisition methods to perform unbiased scans for T1AM metabolites following a single IP injection in mice, we have identified O-sulfonate-T1AM, N-acetyl-T1AM and T1AM-glucuronide as conjugates occurring in vivo, as well as the oxidatively deaminated 3-iodothyroacetic acid and non-iodinated thyroacetic acid. 3-iodothyroacetic acid, O-sulfonate-T1AM and T1AM-glucuronide are present in serum at greater concentrations that unmodified T1AM and all metabolites are extensively distributed to tissues. These results suggest covalent modifications of T1AM may play a critical role in regulating distribution and biological activity of T1AM, and analytical methods to quantify endogenous T1AM should be able to account for these metabolites as well.