© 2021 Sociedad Española de NefrologíaBackground: We aimed to describe the thiamine status in hospitalized hypervolemic heart failure (HF) and/or renal failure (RF) patients treated with furosemide and to investigate whether there was a difference in furosemide-related thiamine deficiency between patients with RF and HF. Methods: Patients who were diagnosed as hypervolemia and treated with intravenous furosemide (at least 40 mg/day) were included in this prospective observational study. Whole blood thiamine concentrations were measured 3 times during hospital follow-up of patients. Results: We evaluated 61 hospitalized hypervolemic patients, of which 22 (36%) were men and 39 (64%) were women, with a mean age of 69.00 ± 10.39 (45–90) years. The baseline and post–hospital admission days 2 and 4 mean thiamine levels were 51.71 ± 20.66 ng/ml, 47.64 ± 15.43 ng/ml and 43.78 ± 16.20 ng/ml, respectively. Thiamine levels of the hypervolemic patients decreased significantly during the hospital stay while furosemide treatment was continuing (p = 0.029). There was a significant decrease in thiamine levels in patients who had HF (p = 0.026) and also, thiamine was significantly lower in HF patients who had previously used oral furosemide before hospitalization. However, these findings were not present in patients with RF. Conclusions: Thiamine substantially decreases in most hypervolemic patients receiving intravenous furosemide treatment during the hospital stay. Thiamine levels were significantly decreased with furosemide treatment in especially HF patients, but the decrease in thiamine levels did not detected at the same rate in RF patients. Diuretic-induced thiamine loss may be less likely in RF patients, probably due to a reduction in excretion.