Tham khảo tài liệu 'food production approaches, challenges and tasks part 13', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Natural Hormones in Food-Producing Animals Legal Measurementsand Analytical Implications 217 simple detection and confirmation of the presence of these semisynthetic versions are not always possible. That is particularly true for estradiol benzoate which has only been detected in hair up to date Duffy et al. 2009 Hooijerink et al. 2005 Rambaud et al. 2005 Stolker et al. 2009 because estradiol benzoate undergoes the loss of the ester group once it reaches the bloodstream. In order to detect the administration of these esterified substances new analytical approaches are required in the same manner that they are required to detect the administration of exogenous hormones in their natural chemical form estradiol testosterone or progesterone . Exogenous natural steroid hormones Hormones of natural origin such as estradiol-17p testosterone or progesterone are still a weak area in residue-monitoring plans due to their endogenous origin as the target compound is always present. In such a case the confirmation of an exogenous administration involves logical difficulties associated with distinguishing an exogenous origin from an endogenous naturally occurring presence of these hormones. The demonstration of an exogenous administration of natural steroids such as testosterone estradiol or cortisol remains problematic. No official threshold has been stated for natural hormone concentrations mainly due to the fact that concentrations of naturally occurring hormones are highly variable and depend on the type of animal product breed gender age disease medication and physiological condition Angeletti et al. 2006 Hartmann et al. 1998 Le Bizec et al. 2009 Pleadin et al. 2011 . The development of methods to provide unequivocal discrimination between the natural presence of an endogenous hormone and its presence as a consequence of an illegal exogenous administration remains a challenge. Some promising analytical approaches have been published in the past few years regarding this
