Circadian changes of clinical chemical and endocrinological parameters.

It is known that all forms of life possess biological rhythms. The structure of this biological daily periodicity is caused by endogenous and exogenous factors. The reasons of circadian periodicity are endogenous processes, which can be modified by exogenous factors. This review describes the daily fluctuations of concentrations or excretions of haematologic, clinical chemical and endocrinological parameters. In blood and serum the largest daily variations are found for the concentrations of haemoglobin, potassium, iron, cortisol, testosterone, somatotropin, prolactin, adrenaline and noradrenaline. In contrast to the serum concentrations of the electrolytes sodium, potassium, calcium and phosphate, their urinary excretions demonstrate pronounced daily rhythms with amplitudes being twice as high as the daily means. There is a remarkable phase shift of 180 degrees between phosphate and the cations both in the serum concentrations and urinary excretions. A daily rhythm comparable to the urinary excretion of electrolytes, has been measured for the urinary excretion of adrenaline, noradrenaline and cortisol. A constant, well defined timing of the sampling is important for those constituents with a marked daily variation. For numerous parameters very contradictory results have been reported in the literature concerning their daily variations, especially in cases of less pronounced rhythms. For some parameters the reasons for the differences in the observed daily variations are discussed.