FSH acts on testicular Sertoli cells to help stimulate sperm production.  Levels of FSH have been shown to be inversely correlated to testicular spermatogenic function.  Schoor et al demonstrated that roughly 90% of men with non-obstructive azoospermia will have an FSH greater than 7.6 mIU/ml or a testicular long axis of 4.6 cm or less.  They also showed that greater than 90% of men with normal sperm production will have an FSH in the normal range. [4]