FNDC5 and COVID-19: Analyzing serum concentrations of irisin, fetuin-A, and FGF-21 were the same in COVID-19 patients with various insulin sensitivity, HOMA-IR value ≤3 and >3, (p = 0.80, p = 0.19 and p = 0.40, respectively), but PTX3 concentration was higher in COVID-19 patients with higher HOMA-IR value than these with lower HOMA-IR value (3038.6 (2310.2–4875.4) vs. 2097.7 (1825.2–3099.4) pg/mL; p = 0.02).