One of the overlooked adverse effects of lithium treatment is neuropsychiatric manifestations induced by hypercalcemia (LAH). Here, we present the case of a patient, with bipolar disorder under lithium, who presented with neuropsychiatric symptoms that revealed hyperparathyroidism-induced hypercalcemia. This case illustrates the importance of monitoring parathyroid function and calcium levels in patients under lithium, especially in premenopaused women. LAH may mimic symptoms of manic episodes and should be sought in the case of brutal onset of confusion, anxiety, and/or hallucinations in the course of a bipolar disorder.