The prevalence of hypomagnesemia was 25.1% in the total study population, 32.4% in males, and 21.5% in females. The occurrence of hypomagnesemia in PHPT has been noted previously and was confirmed in this study; a significant negative correlation was found between serum calcium and magnesium in patients with PHPT. These results are consistent with those of a previous study that included 73 hospitalized patients with PHPT (18). Our results indicate that the higher prevalence of hypercalcemic crisis persisted after adjusting for eGFR and PTH levels in patients with hypomagnesemia. Magnesium levels are maintained within a normal range by a dynamic interplay among intestinal absorption, exchange with bone, and renal excretion, and disruption of these processes may cause hypomagnesemia (26, 27). Magnesium is chiefly eliminated through renal excretion; thus, increased renal excretion leads to hypomagnesemia (28). Although PTH stimulates an increase in tubular magnesium reabsorption in patients with PHPT, hypercalcemia has the opposite effect (15). Hypercalcemia can cause hypomagnesemia owing to increased filtered calcium load in the loop of Henle, resulting in decreased reabsorption of magnesium (29). In the present study, the mean 24 h urinary magnesium levels in the hypomagnesemia group were higher than those in the normal magnesium group in both males and females, but the difference was not statistically significant. This finding may be explained by the fact that we did not measure urinary creatinine and were unable to calculate the renal fractional excretion of magnesium. The renal fractional excretion of magnesium is more effective in evaluating renal magnesium excretion (30).
Hypomagnesemia can also be secondary to impaired intestinal magnesium absorption. A trend of a higher risk of gastrointestinal involvement in the hypomagnesemia group was observed in this study. Moreover, proton pump inhibitors, commonly used in gastrointestinal disorders, have been associated with hypomagnesemia in patients (31, 32). Our study is limited in this aspect because data on the use of proton pump inhibitors were not obtained.
Our results suggest that the typical symptoms of PHPT were more common in the hypomagnesemia group. Patients with hypomagnesemia had a higher prevalence of nephrolithiasis than patients with normal magnesium, which is consistent with the findings of other studies in the general population (33, 34). This may be because magnesium—one of the inhibitors of stone formation—competes with calcium to bind to oxalic acid to form magnesium oxalate, a complex that is more easily soluble in urine (35); at low magnesium levels, this competition is diminished. Hypercalciuria is a well-established risk factor for nephrolithiasis in patients with and without PHPT (36). The mean 24 h urinary calcium levels in the hypomagnesemia group were higher than those in the normal magnesium group in both males and females, but the difference was not statistically significant.
The incidence of bone pain/fractures and osteoporosis was higher in the hypomagnesemia group than in the normal magnesium group. Even after adjusting for potential confounders such as age, sex, BMI, eGFR, and PTH, the association remained essentially unchanged. This result is consistent with the known effect of hypomagnesemia on the prevalence of osteoporosis in the general population (37–39). This is likely because low magnesium can alter trabecular bones owing to the formation of large but fragile crystals (40). Moreover, low magnesium can reduce the vascular supply of bones (41) and increase inflammatory cytokines (42), which promote bone pain and fractures. High PTH activates osteoclasts more readily by enhancing RANKL expression, which increases calcium resorption and bone loss, promoting an osteoporotic state (43). High calcium can cause renal tubular damage—which decreases the renal tubular concentration function—and increases urinary calcium excretion, which leads to polydipsia and polyuria (44). This phenomenon may explain the finding that polyuria was more common in the hypomagnesemia group in our study.
The hypomagnesemia group showed significantly lower average hemoglobin levels than the normal magnesium group in both males and females, and the greater prevalence of anemia in the hypomagnesemia group persisted after controlling for the presence of eGFR and PTH. These results suggest that the effect of hypomagnesemia on anemia in PHPT is independent of the biochemical severity of the disease. The higher prevalence of anemia with hypomagnesemia has been described in individuals without PHPT (32, 45–47).
The present study has certain limitations. First, data on other risk factors for hypomagnesemia were not obtained for our patient population, such as the use of proton pump inhibitors. Second, urinary creatinine and ionized and intracellular free calcium and magnesium levels were not assessed, and effects of magnesium supplementation was not studied. Third, BMD data were available only for approximately three-fifth of the cohort, and distal forearm BMD was not assessed. Fourth, these finding are related to a hospitalized PHPT populations. Thus, these relationships cannot be immediately translated to PHPT outpatients. Lastly, considering the retrospective nature of this study, causal inferences between hypomagnesemia and different clinical features and complications of PHPT cannot be assumed.
Despite these limitations, this study had several strengths. First, this study evaluated data from a large cohort of PHPT patients. Second, blood measurements were performed at the same hospital laboratory. Third, we assessed the prevalence of hypomagnesemia in patients with PHPT. Fourth, this is the first study that assessed whether there is any association between serum magnesium and severity of primary hyperparathyroidism. Finally, we found that hypomagnesemia, which is a frequent electrolyte disorder in PHPT patients, is associated with severity of primary hyperparathyroidism. It is not known whether correcting hypomagnesemia with magnesium supplements in PHPT patients will reduce the progression of PHPT and other associated comorbidities. Further prospective studies with a higher number of patients are needed for this purpose.
In conclusion, the results of our study reveal the effects of hypomagnesemia on patients with PHPT. We found that hypomagnesemia was associated with higher serum calcium and PTH, and clinical symptoms were more common in patients with hypomagnesemia. Hypomagnesemia itself is an independent risk factor for anemia, osteoporosis, and hypercalcemic crisis and may be used as a predictive marker for severity of primary hyperparathyroidism.