Mood-dependent changes of serum lithium concentration in a rapid cycling patient maintained on stable doses of lithium carbonate

Objective Serum lithium levels may be influenced by mood state. We report on a 58-year-old female patient suffering from rapid cycling bipolar disorder. Her serum lithium levels varied greatly, despite stable medication. Methods The patient was observed over a one-year period. Results The patient received a stable medication of lithium carbonate (450 mg), valproate (1500 mg), and clozapine (200 mg). Investigating mood and serum lithium levels over one year revealed six manic and six depressive phases. The mean lithium serum level was 0.67 mmol/L in the depressive states, 0.39 mmol/L in the manic states (t = 4.11, p = 0.001 versus depression), and 0.40 mmol/L in the euthymic states (t = 3.58, p = 0.003 versus depression). Noncompliance was ruled out. The patient gained up to 8 kg during manic phases, accompanied by pretibial edema. Conclusions Changes in serum lithium concentration are probably not caused by altered lithium, but by water metabolism. During mania, body water increases, leading to dilution and therefore a reduction in serum lithium levels. As there is no proof for any other known cause of hypervolemia, we propose the hypothesis that the increase in body water is due to a variant of idiopathic edema.