Objectives Our aim was to compare the radioiodine uptake (RAIU) of congestive heart failure (CHF) patients with that of a control group and investigate the prognostic value. We also assessed correlations of RAIU with other clinical parameters. Methods A total of 32 CHF patients and 14 controls were included in this study. RAIU was measured at the fourth and 24th hour using an uptake probe. The patients were followed up for a mean period of 32.09 +/- 8 months. Results The mean fourth-hour RAIU was 6.28 +/- 5.51 and the mean 24th-hour RAIU was 14.43 +/- 9.49 in the patient group. The mean fourth and 24th hour RAIUs of the control group were 9.93 +/- 1.92 and 22.90 +/- 5.89, respectively, which were statistically higher than that of the patient group (P=0.0001 and 0.001, respectively). Cardiac death occurred in nine patients during follow-up. The fourth and 24th hour uptake ratios were statistically significantly lower in the cardiac death group compared with the no-death group (3 +/- 2.01 vs. 7.57 +/- 5.93 and 7.39 +/- 7.34 vs. 17.18 +/- 8.9, respectively; P=0.003 and 0.007). On stepwise multivariate Cox regression analyses, only urea proved to be a significant independent predictor of cardiac death (hazard ratio 1.021; 95% confidence interval 1.005-1.037; P < 0.008). Significant negative correlations were found between the fourth and 24th hour RAIUs and high-sensitivity C-reactive protein, urea, and creatinine levels, but no correlation was found between RAIUs and estimated glomerular filtration rate. Conclusion CHF patients have lower RAIU values probably because of an expanded iodine pool and prolonged Wolff-Chaikoff effect. Also, patients who died during follow-up had lower RAIU values, but RAIU did not emerge as an independent predictor of death. However, studies with larger patient groups are definitely needed.