Which hormones are important for male infertility?

A man's sperm production is controlled via a complex interplay of hormones in the brain and the testicles.

The control starts in the brain:
The hypothalamus (under the brain) release GNRH, a substance that promotes the pituitary gland to release 2 important hormones, FSH and LH.
FSH travels in the blood to the testicles where it tells certain cells, sertoli cells, to make sperm. When enough sperm is produced, the testicular cells produce inhibin, a hormone that travels in the blood and tells the pituitary gland to stop production and secretion of FSH.
        
LH also comes from the pituitary and travels to the testicles, where it signals to different cells, the Leydig cells to make testosterone.  Testosterone is the male hormone but is not active reproductively until it is converted to dihydrotestosterone via an enzyme, 5-alpha-reductase.  Dihydrotestosterone stimulates male reproductive tract growth and function.  Testosterone and dihydrotestosterone themselves inhibit pituitary production and secretion of LH enough testosterone is made.
 
Prolactin is an pituitary hormone that in men is not directly involved with reproduction.  However, certain medical conditions can cause prolactin to increase, such as pituitary tumors, which in turn causes decreased production of FSH and LH by a variety of mechanisms.            
 
Estrogen, usually thought of as a female hormone, is present in men as well and is called estradiol.  Estradiol levels are typically low, but may be elevated in certain conditions, such as obesity.  Testosterone is converted to estradiol by an enzyme called aromatase.  Aromatase is present in fat cells.
 
By looking at the levels of the various reproductive hormones, especially when analyzed in relation to a comrehensive history and physical examination, we can get a very good sense of a man's reproductive status as well as his prognosis for natural, biological fatherhood.  In addition, there are hormonal conditions that can be treated with medical hormonal manipulation, often resulting in the patience's return to natural fertility.  In addition, a compehensive endocrine assessment wil also detect potentially life-threatening medical conditions that are present in 2% of male fertility patients.