Endogenous overnight creatinine clearance, serum beta 2-microglobulin and serum water during the menstrual cycle

Endogenous overnight (22.00-08.00 hours) creatinine clearance and serum concentrations of beta 2-microglobulin and water were measured three times a week during 11 ovulatory menstrual cycles. In some of the women creatinine clearance changed more than 100% within a week from values below reference range to high normal levels. In all the women the creatinine clearance was higher during the luteal than during the follicular phase and correlated with the production of ovarian hormones. The urinary excretion rate of creatinine was highest during the luteal phase. Urinary volume, serum creatinine and serum water were not significantly influenced by the menstrual phases. An unexplained finding was a parallel change in the individual creatinine clearance and serum beta 2-microglobulin during the luteal, but not during the follicular phase. Our results suggest that ovarian hormones influence creatinine clearance during the menstrual cycle. One must therefore accept even considerable short-time variations in creatinine clearance in fertile women. It remains to be settled if these changes reflect true alterations in glomerular filtration rate or mainly changes in the urinary (tubular) excretion rate of creatinine.