Genetic Conditions

• Conditions like Cushing syndrome, Klinefelter syndrome and Kallmann syndrome can all lead to lower testosterone levels. Varicocele and anatomical defects like undescended testes can also lead to lower testosterone levels.
  
  

Hormone Imbalances

• Depression, obesity, type II diabetes, hyperprolactinemia and pituitary and thyroid disorders can decrease testosterone or increase testosterone antagonists like estrogen.
  
  

Infection, Injury or Illness

• Physical trauma, infection, cancers, tumors, malnutrition, sickle-cell anemia, AIDS, renal failure and cirrhosis of the liver can lead to poor testicle function and lower testosterone levels.
  
  

Drug Abuse

• Long term use of drugs like marijuana, cocaine, oxycodone and alcohol and cigarettes can depress both testosterone and sperm levels, leaving men impotent and infertile.
  
  

Stress

• Chronic lack of sleep or emotional or physical stress can increase levels of cortisol, which leads to fatigue, mental fogginess and low sex drive.