Serum irisin levels and thyroid function--newly discovered association.

Irisin is a newly discovered adipo-myokin, which is reported to have a significant influence on the body metabolism and thermogenesis. Other influencing factors on metabolic state are thyroid hormones, which increase heat production and control the energy balance. Due to numerous similarities in action it seems imperative to explore these substances' potential mutual influence on the body. The aim of the study is to provide the first ever, according to our knowledge, evaluation of serum irisin concentrations in patients with thyroid dysfunction and its correlation with creatine kinase (CK) levels - a serum marker of muscle damage. The studied group consisted of 20 patients with newly diagnosed thyroid disorder - hyperthyroidism or hypothyroidism. Venous blood samples were analyzed for irisin, thyrotropin (TSH), free thyroxine (FT4), free triiodothyronine (FT3) and CK serum concentrations. Patients' body mass index (BMI), body weight and muscle mass were evaluated using bioelectrical impedance analysis (BIA). Median serum irisin levels were lower in hypothyroid (117.30ng/ml) than hyperthyroid (161.95ng/ml) patients with a borderline statistical significance (p=0.0726). The negative correlation between irisin and TSH levels was demonstrated (r=-0.4924, p=0.0230), as well as the positive correlation between irisin and FT4 levels (r=0.4833, p=0.0360). The CK level was negatively correlated with irisin, FT4 and FT3 concentrations (r=-0.7272, p=0.0140; r=-0.9636, p=<0.0001; r=-0.8838, p=0.0007, respectively). The study demonstrates that irisin concentrations may vary according to the thyrometabolic state, which potentially could be related to the degree of muscle damage.