FGF21 and hydrops fetalis: According to the literature, the increased risk of OP in patients with HF is based on certain conditions that are commonly found in this sub-group of patients: oxidative stress, altered RAAS, hyperaldosteronism, renal insufficiency, increased adrenergic drive, reduced testosterone production, secondary hyperparathyroidism, hypovitaminosis D, hyperhomocysteinemia, altered IGFb1 pathway, altered FGF21 pathway and the assumption of medications for HF (Table 1).