Contracting skeletal muscle acts as an endocrine organ, capable of secreting hormones called myokines, such as interleukin 1, Interleukin 15 and Irisin. Myokines are important for regulation of glucose and lipid metabolism. Among these, irisin, is an exercise induced polypeptide, secreted mainly by skeletal muscles and adipose tissue. Irisin is also secreted in small amounts from cardiac muscle, pancreas and sebaceous glands. It is cleaved and secreted from fibronectin type III domain-containing protein 5 (FNDC5 gene) and is regulated by the peroxisome proliferator-activated receptor-γ co activator 1-α (PGC1-α) .
The most important function of Irisin is thermogenesis, by inducing the browning of white adipose tissue, thereby increasing energy consumption, encouraging weight loss and decreasing dietary insulin resistance. Linking Irisin levels with obesity, its mechanism shows that after exercise there is up regulation of a thermogenin ,an Uncoupling Protein-1 (UCP-1) Thyroid hormone also plays an important role in glucose-lipid metabolism and energy homeostasis. Hypothyroidism, a decrease in thyroid hormone levels, can lead to obesity and metabolic syndrome. It has been found that thyroid hormone is also involved in thermogenesis by inducing browning of white adipose tissue, through thyroid hormone receptors. In hypothyroidism, many metabolic processes in the body are affected such as differentiation in the adipose tissue. Increased thyroid stimulating hormone (TSH) causes adipogenesis by stimulating pre-adipocyte differentiation directly through the receptors in the adipose tissue . Fatty tissue is increased by adipocyte hypertrophy as a consequence of previous adipogenesis, and newly formed adipocyte-8, resulting in clinical obesity.
FNDC5 (Fibronectin type III domain-containing protein 5), which is the precursor of irisin, is a protein that is encoded by the FNDC5 gene, is present in many tissues, including the thyroid tissue and therefore effects thyroid hormone.
Dysregulation of both Thyroid and Irisin hormones result in metabolic syndrome. The metabolic syndrome is a common metabolic disorder that has been linked to the increasing prevalence of obesity. By regulating thyroid hormone levels, irisin has a powerful impact on metabolism and thermogenesis. The association of irisin with the thyroid hormone in hypothyroidism still remains unknown and is controversial, and requires further confirmatory studies on experimental models.
Regular exercise is known to have significant beneficial effects on the metabolic health and this is mediated by skeletal muscle. Contracting skeletal muscle is the source of Irisin. Studies have shown an increase in post exercise Irisin levels
