Lithium inhibits the action of fludrocortisone on the kidney.

A patient with autoimmune Addison's disease treated with hydrocortisone and fludrocortisone became mineralocorticoid-deficient whilst taking lithium carbonate for a bipolar illness. During an in-patient metabolic balance study she required 1.0 mg fludrocortisone daily and dietary sodium supplementation to make plasma renin activity and serum potassium normal, and to abolish postural hypotension. We present data to suggest that lithium carbonate inhibits the action of fludrocortisone on the distal renal tubule.