The rate of hypothyroidism is around six times higher in people who take lithium. Low thyroid hormone levels in turn increase the likelihood of developing depression. People taking lithium thus should routinely be assessed for hypothyroidism and treated with synthetic thyroxine if necessary.
Because lithium competes with the antidiuretic hormone in the kidney, it increases water output into the urine, a condition called nephrogenic diabetes insipidus. Clearance of lithium by the kidneys is usually successful with certain diuretic medications, including amiloride and triamterene. It increases the appetite and thirst ("polydypsia") and reduces the activity of thyroid hormone (hypothyroidism). The latter can be corrected by treatment with thyroxine and does not require the lithium dose to be adjusted. Lithium is also believed to permanently affect renal function, although this does not appear to be common.