For example, there is evidence that serum levels of irisin decrease in chronic HF and increase in acute HF due to acute myocardial infarction (Shen et al., 2017; Abd El-Mottaleb et al., 2019), whereas low levels of irisin have been found in T2DM at risk for HF (Akyuz et al., 2021). Here, FNDC5 is linked to hydrops fetalis.