Introduction
Male infertility affects millions of couples worldwide and is responsible for about 40% to 50% of infertility cases in couples^1,2. It is defined as the inability of a man to contribute to conception after one year of regular, unprotected sexual intercourse. This condition has multifactorial causes involving genetic, hormonal, environmental and behavioral factors. Proper assessment of the factors contributing to male infertility is essential to establish effective treatment strategies. Causes of male infertility can be classified as pre-testicular, testicular and post-testicular. Pre-testicular factors include hormonal changes and systemic conditions that impair sperm production. Testicular causes range from congenital diseases to acquired injuries and infections that directly affect the testes^3,4. post-testicular factors involve problems in the transport of sperm⁵. In addition, environmental factors such as exposure to toxic substances, smoking, excessive alcohol consumption and stress are widely recognized as harmful to male reproductive health. Advances in the understanding of the biological mechanisms associated with male infertility have enabled more accurate diagnoses and individualized treatments, increasing the chances of conception^6-9. 

Objectives
This study aims to review the main causes of male infertility, discussing their clinical implications and available therapeutic approaches based on the current scientific literature.

Materials and methods
A bibliographic review of articles published in the PubMed, ScienceDirect and SciELO databases was conducted to support this study.

Discussion
Male infertility results from a series of complex, interrelated factors. Among the pre-testicular factors, hormonal changes involving the hypothalamic-pituitary-gonadal axis stand out. For example, hypogonadotropic hypogonadism can reduce testosterone production and compromise spermatogenesis¹⁰. Other systemic conditions, such as diabetes and autoimmune diseases, also negatively affect male fertility. Testicular causes include congenital anomalies like Klinefelter syndrome, which compromises sperm production. Varicocele, an abnormal dilation of the testicular veins, is one of the main acquired causes of male infertility, leading to increased scrotal temperature and oxidative stress that damages sperm. Infections such as viral orchitis and sexually transmitted diseases can also result in irreversible testicular damage^11,12.

Regarding post-testicular factors, obstructions of the vas deferens and congenital or acquired anatomical abnormalities stand out. Bilateral congenital absence of the vas deferens is often associated with cystic fibrosis. Additionally, ejaculatory dysfunctions, such as retrograde ejaculation, can prevent the proper release of sperm¹³. The impact of environmental and behavioral factors is widely recognized in the literature. Exposure to pesticides, heavy metals and industrial chemicals negatively affects semen quality¹⁴. A sedentary lifestyle, obesity and chronic stress are also associated with reduced fertility. Interventions that promote lifestyle changes have shown benefits in improving semen parameters. With advances in biotechnology, it has become possible to develop advanced assisted reproductive techniques, such as in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), increasing treatment options for men with severe infertility^15,16. However, identifying and treating the underlying causes remain crucial to improving reproductive outcomes¹⁷. 

Conclusion
Male infertility is a complex, multifactorial problem influenced by genetic, hormonal, environmental and behavioral causes. A detailed understanding of these causes is fundamental for accurate diagnosis and the development of effective therapeutic strategies. Detailed clinical evaluation, combined with laboratory and imaging tests, allows for the identification of the specific factors affecting male fertility. Clinical interventions range from hormonal and surgical treatments to assisted reproductive techniques, when necessary. Promoting healthy lifestyle habits and reducing exposure to harmful environmental agents are important preventive measures. Although technological advancements have expanded treatment options, ongoing research is needed to develop more effective and accessible therapies. Despite its challenges, male infertility can often be successfully treated, allowing couples to fulfill their dream of parenthood.

References
1.                Agarwal A, Baskaran S, Parekh N, et al. Infertility in men: Causes, treatments and coping. Current Opinion in Urology 2019.  
2.                Agarwal A, Baskaran S, Parekh N, et al. Male infertility and ART outcomes: Role of advanced semen analysis. Fertility Science and Research 2021.  
3.                Agarwal A, et al. The impact of lifestyle and environmental factors on male infertility. Nature Reviews Urology, 2021.
4.                Barratt CLR, Björndahl L, De Jonge CJ, et al. Male infertility: Advances in diagnosis and treatment. The Lancet 2017.  
5.                Barratt CLR, et al. Male infertility: Causes and consequences. Nature Reviews Urology 2020.
6.                Cocuzza M, et al. Hormonal treatment in male infertility. World J Urology 2018.
7.                Dohle GR, et al. Surgical treatment of male infertility. Best Practice Res Clin Endo Metabolism 2015.
8.                Du Plessis SS, et al. The impact of obesity on male fertility. Asian J Andrology 2015.
9.                Eshre Guideline Group. Management of male infertility. Human Reproduction 2021.
10.             Esteves SC, et al. Management of male infertility: Current trends and future directions. Int J Urology 2020.
11.             Esteves SC, et al. Varicocele and male infertility: Current concepts and future perspectives. Asian J Andrology 2018.
12.             Hamada AJ, et al. The role of oxidative stress in male infertility. Reproductive Biology and Endocrinology 2019.
13.             Hotaling J, et al. Genetic basis of male infertility. Urology Clinics of North America 2020.
14.             Krausz C, et al. Genetic causes of male infertility: New insights and clinical implications. Nature Reviews Urology 2020.
15.             Schlegel PN, Sigman M, Collura B, et al. Azoospermia: Diagnosis and management. Fertility and Sterility 2017.
16.             Tesarik J, et al. Antioxidants and male infertility: From research to clinical practice. Reproductive Bio Medicine Online, 2020.
17.             Winters BR, et al. The role of diet and supplements in male infertility. Urology 2019.