Follistatin (FST) is a secreted glycoprotein that was first identified as a follicle stimulating hormone inhibiting substance in ovarian follicular fluid (1, 2). Human Follistatin cDNA encodes a 344 amino acid (aa) protein with a 29 aa signal sequence, an Nterminal atypical TGF binding domain, three Follistatin domains that contain EGFlike and kazallike motifs, and a highly acidic Cterminal tail. Follistatin is a secreted protein that binds to ligands of the TGF Beta family and regulates their activity by inhibiting their access to signaling receptors. It was originally discovered as activin antagonists whose activity suppresses expression and secretion of the pituitary hormone FSH (follicle stimulating hormone). In addition to being a natural antagonist, follistatin can inhibit the activity of other TGF-Beta ligands including BMP-2, -4, -6, -7, Myostatin, GDF-11, and TGF Beta1. Follistatin is expressed in the pituitary, ovaries, decidual cells of the endometrium, and in some other tissues. Recombinant human Follistatin is a 37.8 kDa protein containing 344 amino acids .
Follistatin and Muscle Growth:
Follistatin works by binding to and inhibiting transforming growth factor-β (TGF-β) peptides such as myostatin which is responsible for regulating and limiting muscle growth. It's also worth pointing out that myostatin may have a regulatory role in skeletal muscle fibrosis; too much myostatin can impair tissue function and cause chronic disease in vital organs, tissues, and bone marrow.
In additional to suppressing the degenerative properties of myostatin, follistatin also suppresses the pituitary gland synthesis and secretion of follicle-stimulating hormone (FSH).  High FSH levels in men may indicate that testicles are not functioning correctly; this condition limits muscle growth, recovery, and normal hormonal function.However, FSH levels that are too low can also negatively impact health and reproductive capabilities.