Most likely it's  Lithium induced diabetes insipidus and it can treated.need to rule out other disorders which cause same issue

Renal toxicity is more common in patients on chronic lithium treatment. Toxicity includes impaired urinary concentrating ability, nephrogenic diabetes insipidus (the most common cause of drug-induced NDI), sodium-losing nephritis, nephrotic syndrome along with other manifestations. The severity of lithium toxicity is often divided into the following three grades: mild, moderate, and severe. Mild symptoms: nausea, vomiting, lethargy, tremor, and fatigue (Serum lithium concentration between 1.5-2.5 mEq/L) Moderate intoxication: confusion, agitation, delirium, tachycardia, and hypertonia (serum lithium concentration between 2.5-3.5 mEq/L) Severe intoxication: Coma, seizures, hyperthermia, and hypotension (serum lithium concentration (less than 3.5 mEq/L)