SKELETAL MUSCLE IS AN ENDOCRINE ORGAN...

Hippocrates observed that “walking is man's best medicine” and thus underscored the benefits of physical activity to health. More than two millennia later, the benefits of physical activity in lowering the risk of death from any cause and improving longevity have been well documented.

Scientists are also beginning to understand the benefits of exercise on a molecular level and to consider skeletal muscle as an endocrine organ, capable of communicating with other tissues through myokines, which are released into the circulation during physical activity.

Adipose tissue communicates with skeletal muscle not only through free fatty acids but also through secretion of various products called adipokines. Adipokines came out as governors of insulin sensitivity and are deregulated in obesity. In addition to well known leptin, adiponectin, interleukin-6 and tumor necrosis factor-alpha, newer adipokines like retinol-binding protein 4 have been associated with insulin resistance. 

There is mounting evidence that not only adipose tissue but also skeletal muscle produces and secretes biologically active proteins or ‘myokines’ that facilitate metabolic crosstalk between organ systems.

 In recent years, increased expression of myostatin, a secreted anabolic inhibitor of muscle growth and development, has been associated with obesity and insulin resistance. Both hypothyroidism and hyperthyroidism affect insulin sensitivity in multiple ways that might overlap adipocyte-myocyte crosstalk. 

Recent studies have provided new insights in effects of processing of the parent hormone T4 to the active T3 at the level of the skeletal muscle.

Summary: Adipocyte-myocyte crosstalk is an important modulator in the development of skeletal muscle insulin resistance. Thyroid disorders are very common and may have detrimental effects on skeletal muscle insulin resistance, potentially by interacting with adipocyte-myocyte crosstalk.