The ROC curve analysis (Figure 4) showed that calculated cut-off points for serum concentration of irisin (against non-HF T2DM for all variables) were 10.4 ng/mL (area under curve [AUC] = 0.95 (95% CI = 0.88–1.00), sensitivity = 81.0%, specificity = 88.0%; Likelihood ratio = 6.881; p = 0.0001) in patients with HFpEF, 8.65 ng/mL (AUC = 0.95, sensitivity = 67.5%, specificity = 99.0%; Likelihood ratio = 11.48; p = 0.0001) in patients with HFmrEF and 8.30 ng/mL (AUC = 0.87, sensitivity = 69.5%, specificity = 94.1%; Likelihood ratio = 12.57; p = 0.0001) in HFrEF patients. Here, FNDC5 is linked to hydrops fetalis.