Characterization of the thyroid hormones level in urine by liquid chromatography coupled to mass spectrometry focus in the antidoping field

Abstract This paper aims to study the metabolism of thyroid hormones (TH) in urine by liquid chromatography–tandem mass spectrometry (LC‐MS/MS). The method was applied to samples collected before and after the administration of sodium triiodothyronine (T3) and sodium levothyroxine (T4) to a euthyroid volunteer and to samples of athletes declaring and not declaring thyroid supplementation. Samples were analyzed by LC‐MS/MS after enzymatic hydrolysis, liquid–liquid, and solid‐phase extractions. Ratios between T3/thyronine and T4/3,3′‐T2 may be used for the detection of the administration of exogenous T3 in urine. Meanwhile, 3‐T1 concentrations may be used to detect exogenous T4 administration. Nevertheless, these markers may not work properly in hypothyroid population, as athletes seem to be. The levels of T3 and T4 of athletes were lower than those of a euthyroid state even when they are under administration of TH supplements. The HTP axis high efficiency does not allow observing differences between athletes who do not declare and those who declare having used TH supplementation by direct measurements of T3 and T4 in urine. The detection of TH administration in urine (triiodothyronine and levothyroxine) may work when dealing with euthyroid individuals. Nevertheless, in individuals with hypothyroidism where the tendency is toward the maintenance of homeostasis, and it may be not possible to detect their consumption by applying cut‐off values.