There are three major causes of azoospermia: hormonal, the testis are not properly stimulated; production problems in the testis or complications in the carriage of of sperm from the testicles to the urethra.
Azoospermia, in this case, is originated by altered hormone levels, especially the FSH (follicle stimulant hormone), responsible of stimulating the testicle so that it can produce sperm.
Patients that have secretory azoospermia due to a cause of this type will have a slightly lower testicular volume, their testicles will not produce sperm and will present low FSH levels.
Several situations can trigger a hormone imbalance, from genetic diseases, like the Klinefelter Syndrome, to the use of drugs that alter the hypophysis and its normal hormone secretion.
A trauma or testicular infection may be the cause of testicular problems, spermatogenesis, under these circumstances, is not possible.
Patients with testicular causes will also suffer secretory azoospermia, although the symmptoms will differ from those induced by pre-testicular factors. They have a diminished testicular volume (due to the lack of sperm production), but FSH levels are high. This is a byproduct of a the unresponsiveness of the testicles to normal levels of FSH, forcing the organism to produce it at higher quantities.
Any form and sort of obstructive azoospermia has a post-testicular cause. The testis are actually able to produce sperm (and to it in the proper manner), but the transportation of sperm to the urethra fails. There is a problem either in the efferent ducts or in the epididymis, as their function is to carry the sperm to the urethra.
The obstruction can be induced by many causes; congenital (the patient was born without efferent ducts); disease-related (illnesses like meningitis or mumps can alter the development of the ducts), or even traumas and surgical interventions might damage these structures.
Patients will have a normal testicular size (the testicle has its corresponding number of cells and a good sperm production) and FSH levels will be normal as well. Obstructive azoospermia is the least serious form of azoospermia. The sperm can be extracted by aspiration. Besides, there are more therapuetic alternatives to address it.