It is not unusual for there to be a difference in size between the testicles, but this is usually no more than 25%. If one testicle is markedly larger than the other, it is possible that one is too small or the other too large. A man can function sexually and father children quite successfully with only one testicle.
A small testicle can be a developmental abnormality caused by a poor blood supply to one testicle or it can be caused by an injury or irradiation (eg. for cancer) to the testicle at a later time.
Serious infections of a testicle (orchitis) may damage the gland and cause it to scar and shrink.
Torsion of the testis is a medical emergency in which the testicle twists around and cuts off the blood vessels that supply it. Pain occurs in both the testicle and the groin. If surgery is not performed within twelve hours, the testicle will die and shrink in size.
Other less common causes of a small testicle include:
- an underactive pituitary gland (controls every other gland in the body),
- poorly controlled diabetes (damages small blood vessels that supply the testes),
- cirrhosis (damage to the liver that causes abnormal levels of hormones),
- excess thyroxine from the thyroid gland in the neck (hyperthyroidism),
- Klinefelter syndrome (additional X chromosomes matched with a single Y chromosome)
- tuberculosis (TB) infection of a testicle.
As a side effect, some drugs may cause the testicles to become smaller and function poorly. Examples include cell destroying drugs (cytotoxics) used to treat cancer, spironolactone (fluid remover), digoxin (for heart disease), narcotics (for pain), ketoconazole (for fungal infections), alcohol and marijuana.