In patients with mild HF, serum FGF21 level was significantly negatively correlated with NT-proBNP levels (ρ = −0.37, p = 0.0022), E/e’ ratio (ρ = −0.29, p = 0.0182), TR velocity (ρ = −0.24, p = 0.0470) and RV-RA gradient (ρ = −0.24, p = 0.0472). The gene discussed is NPPB; the disease is hydrops fetalis.