Renal function impairment and electrolyte disorders in hypothyroidism are frequently subtle and rarely observed in clinical practice. To assess the extent of these defects, serum concentration of electrolytes and glomerular filtration rate were estimated before and after thyroid replacement therapy in 41 patients with primary hypothyroidism. All patients had decreased glomerular filtration rates and 22 patients had increased serum creatinine levels. Although a relationship between creatinine clearance and serum thyrothropin-stimulating hormone was not found, a weak correlation between age and serum creatinine concentration was observed. Hyponatremia was documented in 45% of 22 patients with elevated serum creatinine but in only 21% of 19 patients with normal creatinine. All these defects were corrected by treatment with thyroid hormone. We conclude that creatinine clearance was slightly decreased in all patients with hypothyroidism, this decrease being more noticeable in elderly patients. The greater the impairment in renal function, the more common was the occurrence of hyponatremia.