Testicular Tumor: A testicular tumor, such as a Leydig cell tumor, can produce excess testosterone, leading to elevated serum levels. This is a common cause of endogenous testosterone excess in adult males.

Congenital Adrenal Hyperplasia (CAH): CAH is a group of genetic disorders that affect the adrenal glands, leading to an overproduction of androgens, including testosterone. Although typically diagnosed in childhood, some forms of CAH may not be diagnosed until adulthood.
Adrenal Tumor: An adrenal tumor, such as an adrenal adenoma or carcinoma, can produce excess androgens, including testosterone, leading to elevated serum levels.
Hyperthyroidism: Hyperthyroidism can increase the production of sex hormone-binding globulin (SHBG), leading to an increase in total testosterone levels. However, free testosterone levels may be normal or slightly elevated.
Obesity: Obesity is associated with increased aromatization of androgens to estrogens, which can lead to an increase in SHBG and, subsequently, an increase in total testosterone levels.

Cushing's Syndrome: Cushing's syndrome is a rare endocrine disorder caused by excess cortisol production. While not typically associated with elevated testosterone, some cases of Cushing's syndrome can present with androgen excess, including elevated testosterone levels.
Pituitary Tumor: A pituitary tumor, such as a gonadotropin-secreting adenoma, can stimulate the testes to produce excess testosterone, leading to elevated serum levels.

Familial Glucocorticoid Resistance: This is a rare genetic disorder characterized by resistance to glucocorticoids, leading to an overproduction of androgens, including testosterone.
17,20-Desmolase Deficiency: This is a rare genetic disorder that affects the production of androgens, leading to an overproduction of testosterone.
Gonadotropin-Independent Precocious Puberty: This is a rare condition characterized by premature puberty in males, often caused by a testicular tumor or other rare genetic disorders.