PTH and hyperparathyroidism: Palermo et al. [29] revealed an interplay between irisin and PTH; they reported myotubes treated with PTH (1-34) had decreased FNDC5 expression and r-irisin administration led to a 50% downregulation of parathyroid hormone receptor (PTH-r) mRNA expression compared to untreated cells (P < 0.001); they also found that irisin levels were prominently inclined to a lower direction in the hyperparathyroidism (PHPT) group, rather than the age and BMI matched controls (P < 0.001) but no apparent bidirectional and internal BMD-irisin correlation was observed.