Renal function is profoundly influenced by thyroid status; however, this has not been studied in detail in human subjects. The purpose of the present study was to determine the relationship between renal function and thyroid status before and after treatment for hypothyroidism and hyperthyroidism, respectively. In 37 consecutive hypothyroid and 14 hyperthyroid patients renal function as measured by plasma creatinine and glomerular filtration rate (GFR) [based on the modification of diet in renal disease (MDRD) formula] was determined before treatment and after regaining euthyroidism. Renal function improved significantly during treatment of hypothyroidism and decreased during treatment of hyperthyroidism. There was a strong correlation between the change in thyroid status determined as the ratio log(10)(fT4 post-treatment/fT4 pretreatment) and the change in renal function as a result of therapy expressed as serum creatinine (r(2) = 0.81, P < 0.0001) and estimated GFR (0.69, P < 0.0001). The kidney is an important target of thyroid hormone action.