Use of immunochemical methods for hormonal determinations makes endocrinologic diagnosis faster and easier; but insufficient information on possible false elevated or falsely low values may lead to potentially harmful and irreversible effects for the patient. In addition to the well known negative effects on all laboratory determinations of pre-analytical errors, the occurrence of abnormal concentrations of some proteins or the presence of therapeutic drugs is well known. Immunochemical methods are negatively influenced by cross-reactions, as well as by the interference caused by the presence of heterophilic antibodies and the "effect of high dose". Such effects not only make the results obtained by different laboratories incomparable, but also may lead to obtaining results of hormonal determinations completely inconsistent with clinical observations.