Urinary magnesium, calcium, and phosphate excretion during magnesium sulfate infusion.

Urinary excretion of magnesium, calcium, and phosphate was measured in 20 preeclamptic pregnant women treated with intravenous magnesium sulfate. Blood levels of these substances, as well as of parathyroid hormone and calcitonin, were also measured. Urinary magnesium excretion increased 20-fold during magnesium sulfate infusion. Seventy-five percent of the infused magnesium was excreted during the infusion, and 90% within 24 hours after treatment. Urinary calcium concentration increased 4.5-fold during magnesium sulfate infusion, and the urinary calcium excretion rate was 3 times that observed in controls. Urinary phosphate concentration and excretion rate were the same in treated and control subjects. During treatment, maternal serum total calcium fell 25%, ionized calcium fell 12%, and parathyroid hormone rose 26%. Magnesium sulfate therapy depresses maternal calcium levels by increasing urinary calcium loss, and increased parathyroid hormone output prevents more marked hypocalcemia.