Follistatin is a single-chain gonadal protein that specifically inhibits follicle-stimulating hormone release. The single FST gene encodes two isoforms, FST317 and FST344 containing 317 and 344 amino acids respectively, resulting from alternative splicing of the precursor mRNA. There is evidence linking polycystic ovary syndrome and follistatin. 

Follistatin binds directly to activin and functions as an activin antagonist. It specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone.