Magnesium Infusions and Calcium Levels
Magnesium infusions decrease, not increase, serum calcium levels. This hypocalcemic effect occurs through multiple mechanisms and is well-documented in both clinical guidelines and research studies.
Mechanism of Magnesium-Induced Hypocalcemia
Magnesium sulfate infusion causes a gradual and progressive decrease in corrected serum calcium levels 1. The hypocalcemia results primarily from:
- Increased renal calcium loss – Magnesium infusion causes hypercalciuria, with urinary calcium excretion gradually increasing and reaching a peak between 120-180 minutes after infusion begins 1
- Suppression of parathyroid hormone (PTH) secretion – Hypermagnesemia directly inhibits PTH release, though this mechanism appears to play only a partial role in the early phase of hypocalcemia 2, 1
- Decreased nephrogenous cAMP excretion – This falls rapidly during magnesium infusion, reaching a nadir between 60-120 minutes, indicating suppressed parathyroid function 1
Clinical Evidence from Preeclampsia/Eclampsia Treatment
In patients receiving magnesium sulfate therapy for severe preeclampsia/eclampsia, mean baseline ionized calcium levels of 3.8±0.5 mg/dL did not change significantly during magnesium infusion 3. However, this finding contrasts with other studies showing hypocalcemia, and severe symptomatic hypocalcemia with low plasma PTH concentration has been documented following therapeutic magnesium sulfate use 2.
A case report documented prominent hypocalcemia and hyperkalemia attributed to magnesium infusion in a preeclamptic patient, illustrating that iatrogenic hypermagnesemia is an underrecognized cause of these electrolyte disturbances 4.
Clinical Implications and Monitoring
The FDA-approved indication for Magnesium Sulfate Injection includes replacement therapy in magnesium deficiency, especially acute hypomagnesemia accompanied by signs of tetany similar to hypocalcemia 5. This indication acknowledges the complex relationship between magnesium and calcium homeostasis.
When magnesium infusions are used, close electrolyte monitoring is necessary, particularly for calcium and potassium levels 4. The hypocalcemic effect is primarily due to magnesium itself rather than the sulfate component, as demonstrated by control studies using sodium sulfate 1.
Important Caveats
- Magnesium does not appear to exert its therapeutic effect in preeclampsia/eclampsia by substantially modulating serum ionized calcium 3
- The effect of exogenous magnesium on intracellular calcium cannot be ruled out, even when serum ionized calcium remains stable 3
- Serum magnesium levels achieved during standard infusion protocols (5g IV bolus followed by 1g/hr) range from 4.5-5.9 mg/dL, which is well above the therapeutic range of 4-7 mg/dL 6
In pediatric parenteral nutrition, premature newborns exposed to maternal magnesium sulfate therapy may have high magnesium levels in the first days of life, and their low postnatal glomerular filtration rates limit their ability to excrete excessive magnesium 7. Therefore, magnesium intakes must be limited and adapted to postnatal blood concentrations in these infants 7.