Abstract
The limited value of plasma measurements in the management of treatment with lithium is discussed in the light of the mechanisms of its therapeutic actions and toxic effects.The plasma level of lithium usually rises twofold or threefold in the three to five hours after ingestion of each dose of delayed-release tablets and then gradually falls. The precise shape and height of the lithium curve depend on gastric emptying, which can be slowed with propantheline or speeded with metoclopramide. Depressed or demented patients may be irregular in taking their tablets and variable in food intake. Both the time of the blood test and this behaviour must be considered before changing the prescribed dose of lithium salt because of a laboratory result. A lithium tolerance curve may be a safer guide to treatment than single measures.Mild intermittent thirst is a common early side effect, and severe persistent thirst with polyuria is an uncommon later effect of daily intakes of at least 1,500 mg lithium carbonate. This diabetes insipidus is reversible, non-progressive, unrelated to plasma level, and distinct in attack from lithium-induced hypothyroidism, which may occur at low dosage but is also usually of late onset and reversible or treatable with thyroxine while lithium is continued. Obesity is another occasional effect of large doses. These side effects and the antimanic and prophylactic effects may have different mechanisms.