Plasma levels of antidiuretic hormone in patients receiving prolonged lithium therapy.

Among 18 patients receiving prophylactic, long-term lithium treatment for manic-depressive psychosis, a high incidence of increased thirst and frequency of micturition (60-70 per cent) was noted on direct questioning. Symptoms arose at varying times after the start of lithium therapy; in no patient did symptoms antedate the use of the drug. Plasma levels of antidiuretic hormone were found, on average, to be higher than in normal control subjects for a given level of plasma osmolality, although the scatter of results was wide. It is suggested that elevation of antidiuretic hormone occurs as a compensatory mechansims for the polyuria which is a common feature of long-term lithium treatment. The more florid form of nephrogenic diabetes insipidus occasionally seen in lithium takers seems likely to be due to a different mechanism from the more common mild polyuria.