Role of Serum Lithium Concentrations in Predicting Hyperparathyroidism and Hypercalcemia

<b>Purpose:</b> Lithium (Li), which is extensively used in the treatment of mood disorders such as bipolar disorder, has been associated with hyperparathyroidism. However, the relationship between the serum Li concentration and hyperparathyroidism remains unclear. This study aimed (1) to investigate the incidence of hyperparathyroidism and hypercalcemia in consecutive patients treated with Li, (2) to assess the correlation between serum Li concentration and hyperparathyroidism/hypercalcemia, and (3) to establish cutoff values for predicting hyperparathyroidism and hypercalcemia based on serum Li concentration. <b>Methods:</b> This retrospective cross-sectional study was conducted at the Department of Psychiatry and Department of Medicine at Tokyo Women's Medical University. Ninety-seven consecutive individuals without renal impairment and with an estimated glomerular filtration rate (eGFR) equal to or greater than 45 mL/min/1.73 m² were included. <b>Results:</b> Hyperparathyroidism and hypercalcemia were observed in 35.1% and 9.3% of the patients on Li, respectively. The serum Li concentration showed a significant positive correlation with hyperparathyroidism and hypercalcemia, independent of other factors. The cutoff values for predicting hyperparathyroidism and hypercalcemia were 0.52 and 0.62 mEq/L, respectively. <b>Conclusions:</b> This study confirmed that the high incidence of hyperparathyroidism and hypercalcemia in patients treated with Li. Clinicians should be aware that Li treatment may induce hyperparathyroidism, and a serum Li concentration exceeding 0.52 mEq/L may pose an increased risk. Monitoring serum calcium and Li concentrations is recommended in patients undergoing Li treatment.