FNDC5 and acromegaly: In patients with acromegaly, irisin was negatively correlated with fasting insulin (r = −0.367; p = 0.042), HOMA-IR (r = −0.510; p = 0.011), Castelli I (r = −0.416; p = 0.005), Castelli II (r = −0.400; p = 0.001), and AC (r = −0.417; p = 0.005) whereas in controls, irisin correlated significantly only with age (r = −0.429; p = 0.016) (Table 5).