A case of a woman with severe lithium toxicity and hypothyroidism with persistent hyponatremia and hypokalemia is presented. The mechansisms of the development of lithium toxicity by alterations in fluid and sodium metabolism, including those induced by lithium itself, are reviewed. The possibility that hypothyroidsm may have been a significant determinant in the development of this case of lithium toxicity is considered and discussed. This is of special interest given that lithium can induce hypothyroidism. In addition, the safe reinstitution of lithium prophylaxis in this woman can be reported.