Lithium-associated hyperparathyroidism and hypercalcaemia: a case-control cross-sectional study.

Background: Lithium is recommended as a first-line treatment for Bipolar Disorder (BD). Thyroid and renal alterations are well known lithium side-effects, while effects on parathyroids are less studied. The aim of this case-control cross-sectional study is to compare parathyroid hormone (PTH) and calcium levels in lithium-exposed bipolar patients and in subjects who had never been exposed to lithium.

Methods: 112 BD patients were enrolled, 58 on lithium since at least 1 month (mean exposure 60.8 ± 74.8 months) and 54 in the control group. Blood exams included complete blood count, PTH, total and ionized calcium, TSH, T3 and T4, creatinine, urea, sodium and potassium, and lithium serum levels. The Student's t-test and the Pearson's Chi-square test were used for bivariate analyses. A linear regression model was used to analyze the relationship between the duration of exposure to lithium and PTH and calcium levels.

Results: PTH and ionized calcium levels were significantly higher in lithium-exposed patients; the proportions of subjects with hyperparathyroidism (8.6%) and hypercalcaemia (24.1%) were significantly greater in lithium-exposed patients. The linear regression analyses showed a significant effect of exposure to lithium in months on ionized calcium levels but not on PTH levels.

Limitations: Given the cross-sectional design of the study we could not identify the exact time of occurrence of hyperparathyroidism.

Conclusions: Our results indicate that lithium-associated stimulation of parathyroid function is more common than assumed to date. Among parameters to be evaluated prior to lithium implementation and during long-term lithium maintenance, calcium (and eventually PTH) should be added.