The aim of this study was to analyze the prevalence, clinical significance and prognostic implications of alterations in thyroid function tests (TFTs) in patients with acute kidney injury (AKI).A prospective study was carried out in patients hospitalized for AKI for 2 consecutive years. TFTs (serum thyrotropin [TSH], free thyroxine [FT4] and total triiodothyronine [T3] concentrations) were completed for each patient on 3 occasions: at admission, at hospital discharge and at their first outpatient visit. TFTs were related to clinical and analytical data. Thirty-five patients (16 women [45.7%], mean age ± SD, 65.2 ± 18.0 years) with AKI (creatinine 5.6 ± 2.2 mg/dL) were studied. There were 10 (28.6%), 10 (28.6%), 11 (31.4%) and 4 (11.4%) patients with prerenal, renal, mixed (prerenal and renal), and postrenal AKI, respectively.Total prevalence of alterations in TFTs was 82.9% (n=29). Of those, euthyroid sick syndrome (ESS) with low T3 only was the most common (n=13, 37.1%) derangement. In the whole group of patients, median TSH (0.93 µU/mL, interquartile range 0.35-2.27 µU/mL)and mean FT4 (1.2 ± 0.3 ng/dL) were normal, whereas mean T3 was low (0.7 ± 0.1 ng/mL). TSH, FT4 and T3 were similar in different types of AKI. On simple regression analysis, we found a negative correlation only between TSH and serum urea concentrations (ro=-0.382; p=0.024). At hospital discharge (median hospital stay 6 days; range 2-10 days), TFT showed significant changes only in T3 concentrations (0.8 ± 0.3 ng/mL, p=0.013). At this point, the percentage of patients with normal TFT increased from 17.1% at baseline to 40% at discharge and then to 66.7% at their first outpatient visit. We found no association between the presence and type of alterations in TFT and clinical factors (sex, age, personal history of diabetes and/or hypertension, number and type of drugs used, number of signs and symptoms at AKI diagnosis, and degree, type and cause of AKI) or prognostic factors (hospital stay, recovery of renal function, need for renal replacement therapy by hemodialysis, development and degree of residual chronic renal failure and mortality) associated with AKI.Over 80% of AKI patients exhibit alterations in TFT. The commonest derangement is ESS (~70%), mainly low T3 syndrome, which is present in about one third of the patients with altered TFT. ESS recovers spontaneously as renal function improves. The presence of TFT alterations seems to not be associated with clinical and prognostic implications in AKI patients.