We investigated the plasma concentrations of thyroid hormones in 27 patients with acute renal failure. Both the mean free T3, 1.17±0.74(SD) pmol/L (range 0.5–2.9 pmol/L) and total T3 plasma concentrations, 0.43±0.14 nmol/L (range 0.3–0.7 nmol/L) were reduced compared to the normal range (3.3–8.2 pmol/L and 1.0–3.0 nmol/L, respectively). In all 15 patients with plasma creatinine>3.33 mg/dL (295μmol/L), the mean FT3 was 0.92 pmol/L and the plasma concentrations have all been<1.5 pmol/L. There was a weak correlation between the plasma creatinine and the plasma concentrations of TT3 (x = 0.38, p = 0.049). Similarly, both plasma free T4 and total T4 were also reduced in the patients compared to the normal range: mean free T4 5.91±3.68 pmol/L, range 1.0–13.1 pmol/L, and total T4 47.80±29.31 nmol/L, range 5–99 nmol/L; normal range 9.4–25.0 pmol/L and 50–160 nmol/L, respectively. There was no difference between the ranges in either plasma TSH, patients 1.19±1.25 mU/L, range 0.1–4.9 mU/L, vs. control range 0.3–6.0 mU/L; and TBG, patients 19.05 mg/L±3.69, range 11.0–26.9 mg/L, controls range 13.0–30.0 mg/L. Thus we have determined that patients in the early phase of ARF have consistently reduced plasma concentrations of both free and total T3, and reduced concentrations of free and total T4; and that the reduction of TT3 is dependent upon the plasma creatinine level.