Blood was sampled from nine free-ranging white whales (beluga whale, Delphinapterus leucas) from Svalbard, Norway during the summers of 2013 and 2014. Total concentrations of eleven thyroid hormones and metabolites were measured in serum using a novel liquid chromatography tandem mass spectrometry analytical method. Measurements of these compounds in plasma gave the same results as in serum. The three hormones found in highest concentrations were 3,3′,5-triiodothyronine (T3), 3,3′,5′-triiodothyronine (rT3) and thyroxine (T4). Traces of associated metabolites were also found.