[Circadian rhythms in endocrinology].

The function and fission properties of every cell in the organism follow a distinct rhythm. The synchronization of all these peripheral rhythms is controlled by a superordinate "master-clock", which is located in the nucleus suprachiasmaticus of the hypothalamus. Endocrine systems, aside from the autonomic nervous system, serve as efferent signals to transmit the rhythm to peripheral organs. The measurement of time-dependent changes in circulating hormone concentrations allows conclusions concerning the functionality of circadian rhythms and therefore represents a central approach in endocrine research. Regarding the interpretation of diurnal changes, one has to keep in mind that apart from the light/dark cycle other factors like the sleep/wake cycle or food intake also clearly influence the secretory activity of many endocrine organs. The dissociation of intrinsic circadian rhythms from other influencing factors requires the performance of costly and well-controlled experimental studies. Moreover, because of the complex interactions between hormones it is necessary to assess diurnal changes in activity of various endocrine systems in parallel. Only such an approach enables to understand the impact of endocrine rhythms on the complexly regulated homeostasis of e.g. glucose metabolism or the immune system.