Lithium clearance was studied to investigate proximal tubular function in patients with hyperthyroidism (n = 10) and control subjects (n = 7). Patients with hyperthyroidism showed significantly reduced fractional excretion of lithium (FELi) compared with control subjects (15.0 ± 1.5%, n = 10, vs. 23.7 ± 0.6%, n = 7, means ± SE, p < 0.001). The reduced FELi of the hyperthyroid state was reversed toward control values with treatment by antithyroid drugs (12.6 ± 2.6 toward 26.8 ± 2.5% for 5 patients, means ± SE). Tubular reabsorption of phosphate (TRP) was significantly increased in hyperthyroid patients compared with control subjects (96.1 ± 0.7 vs. 87.5 ± 0.7%, p < 0.001), and it returned to control values after the treatment. Our data demonstrate that lithium clearance is decreased and TRP is increased in patients with hyperthyroidism, which suggests that proximal tubular reabsorption of sodium and TRP is increased in hyperthyroidism.