Does thyroid disease affect male fertility or not? The first thing to understand is the function and role of the thyroid gland. The thyroid gland is located in the neck below the thyroid cartilage, on both sides of the trachea, and is the largest endocrine organ in the body. The main function of the thyroid gland is to synthesize, store and secrete thyroid hormones, regulate human metabolism, produce heat and promote growth and development. Thyroid disorders are common in clinical practice and include thyroid tumors, hyperthyroidism (hyperthyroidism), hypothyroidism (hypothyroidism), and thyroid cancer. Among them, the two main types of diseases related to male reproduction are hyperthyroidism and hypothyroidism. Hyperthyroidism is a collective term for a series of endocrine disorders caused by excessive secretion of thyroid hormones in the body due to various reasons, resulting in increased excitability and hyper-metabolism in the nervous, circulatory and digestive systems of the body. The effects of hyperthyroidism on the male reproductive system are mainly hypoactive sexual desire, erectile dysfunction and abnormal semen quality, all three of which are closely related to male fertility. The reason may be that hyperthyroidism causes disturbance in the balance of hypothalamic-pituitary-testicular axis system in men. Clinically, luteinizing hormone (LH), follicle stimulating hormone (FSH), zero ketone (T), and estradiol (E2) are increased in the blood of male hyperthyroid patients. When FSH and LH are elevated, the spermatogenic function of the testes is affected, which causes poor semen parameters, and when estradiol is increased, it reduces male sexual desire. However, when hyperthyroidism is stabilized with treatment, many patients with abnormal semen will return to normal. Hypothyroidism is a clinical syndrome caused by a decrease in thyroid hormones and a decrease in the function of the body’s metabolic systems due to different causes. It may have comorbidities such as abnormal lipid metabolism, abnormal glucose metabolism, cardiovascular pathology and abnormal sex hormones. Hypothyroidism may cause a decrease in testosterone synthesis in the testes. Sperm production is closely related to testosterone levels, and sperm production will be inhibited when human testosterone synthesis is reduced, resulting in a decrease in sperm concentration and vitality. Hypothyroidism can also cause erectile dysfunction, which can also have a negative impact on male fertility. Most patients with hypothyroidism have normalized semen quality after thyroxine supplementation to correct the condition. Thyroid disorders can affect the quality of male semen and sexual function, causing infertility, but early diagnosis and treatment can restore the quality of semen to normal for most patients after the condition has recovered and they can conceive and raise offspring.