Participants are randomly assigned to receive lithium capsules or placebo (look-alike capsules with no active ingredient). They follow a low-iodine diet for 2 weeks before starting treatment and are then admitted to the NIH Clinical Center for study and treatment for 11 days, during which they remain on the low-iodine diet. Blood samples are collected almost every day to analyze thyroid hormones, kidney and liver function, lithium concentrations and other tests.