The association between irisin and muscle metabolism in different thyroid disorders

Irisin is a new adipo-myokine, encoded by the FNDC5 gene. Currently, there is a discussion regarding the relation between thyroid function and irisin concentration. This prospective study assesses the influence of thyrometabolic changes on serum irisin concentration in association with altered muscle metabolism. This is performed on a large cohort of patients affected by severe hypo- or hyperthyroidism, as well as by the expression of the FNDC5 gene in thyroid tissue affected by different pathologies.

The study group comprised 119 patients with newly diagnosed severe hyperthyroidism or hypothyroidism, and a control group of 45 healthy subjects. Body composition, serum irisin concentrations, and thyroid-related hormones, creatine kinase, dystrophin and titin concentrations were evaluated. FNDC5 expression was also analysed in tissue samples from 80 patients with nontoxic multinodular goitre, toxic goitre, Graves' disease and papillary thyroid cancer.

Irisin concentration was lower in patients with prolonged hypothyroidism. There was a tendency towards lower dystrophin and titin concentrations in patients with hypothyroidism and hyperthyroidism. Restoration of euthyroidism in patients with hypothyroidism resulted in a decreased muscle mass with an increase in irisin concentrations, while the hyperthyroid group showed an increase in fat mass. Statistically significant overexpression of FNDC5 gene was found in patients with toxic goitre as compared to Graves' disease, papillary thyroid cancer and controls.

The presented data support the theory that irisin concentration changes are associated with prolonged hypothyroidism and might primarily constitute the result of prolonged myopathy. These changes are most likely not related to the expression of the FNDC5 gene in the thyroid gland.