Variations in Plasma Electrolytes and Thirst Perception During the Menstrual Cycle

Introduction: Electrolyte-dependent water retention associated with hormonal changes during the menstrual cycle may alter the normal reference intervals of plasma electrolytes with important effects on central nervous system, cardiovascular and renal functions. Hence, we sought to determine and compare plasma electrolytes (sodium, potassium, bicarbonate, chloride and urea) concentrations and thirst perception during different phases of the menstrual cycle. Methods: Blood samples obtained from the anti-cubital veins of 40 healthy females (18 – 25 years, 27 - 33 days menstrual cycle) on the 2nd (early follicular/menstrual, n = 10), 8th (late follicular, n = 10), 14th (ovulatory, n = 10) and 22nd (luteal, n = 10) day of the menstrual cycle were analysed for plasma electrolytes concentrations by photoelectric flame photometry. Thirst perception (TP) ratings were also obtained at the time of sample collection using the Visual Analogue Scale. Results: Plasma [Na + ] ( P = 0.003) and [Cl - ] ( P = 0.02) were highest during ovulation while the late follicular and luteal phases had the least [Na + ] and [Cl - ] respectively. Plasma [K + ], [HCO 3 - ], and [urea] did not differ significantly in relation to the phase of menstrual cycle. In addition, TP increased in a phase-dependent non-significant manner until after ovulation. Conclusion: Plasma concentrations of sodium and chloride are increased during ovulation. This may be attributed to oestrogen-related salt retention, whereas the lower sodium and chloride concentrations observed in the luteal phase (compared to ovulation especially) could be due to the natriuretic action of progesterone.