The levels of irisin were significantly higher in HFpEF patients than in HFrEF individuals (7.90 ng/mL; 95% CI = 6.85–10.66 ng/mL vs. 3.41 ng/mL; 95% CI = 2.80–4.24 ng/mL, p = 0.001) and were also higher than in HFmrEF patients (3.95 ng/mL; 95% CI = 3.10–4.75 ng/mL, p = 0.001), whereas non-HF T2DM patients (12.9 ng/mL; 95% CI = 11.2–13.4 ng/mL) demonstrated lower levels of irisin when compared to healthy volunteers (15.1 ng/mL; 95% CI = 13.6–16.7 ng/mL; p = 0.001) (Figure 2). The gene discussed is FNDC5; the disease is hydrops fetalis.