Gynecomastia: This condition, characterized by the enlargement of breast tissue in males, can lead to nipple discharge due to hormonal imbalances, particularly an increase in estrogen relative to testosterone. It's a common condition and can be physiological or due to various pathological conditions.

Prolactinoma: A type of pituitary tumor that secretes prolactin, leading to an increase in prolactin levels, which can cause nipple discharge in men. Other symptoms might include headaches, visual disturbances, and sexual dysfunction.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can lead to an increase in thyrotropin-releasing hormone (TRH), which stimulates the secretion of prolactin, potentially causing nipple discharge.
Medication-Induced: Certain medications, such as antipsychotics, antidepressants, and drugs used to treat hypertension and gastrointestinal disorders, can cause nipple discharge as a side effect by affecting hormone levels.

Breast Cancer: Although rare in men, breast cancer can present with nipple discharge, often unilateral and bloody. It's crucial to investigate any nipple discharge in men thoroughly to rule out malignancy.
Pituitary Macroadenoma: A larger pituitary tumor that can cause an increase in prolactin levels, leading to nipple discharge, and may also compress surrounding structures, leading to other symptoms like visual field defects.

Hypogonadism: Conditions that lead to low testosterone levels can sometimes cause an imbalance in hormone levels, potentially leading to nipple discharge, though this is less common.
Choriocarcinoma: A rare type of testicular cancer that can produce human chorionic gonadotropin (hCG), leading to an increase in estrogen levels and potentially causing nipple discharge.
Neurogenic Causes: Rare conditions affecting the nervous system, such as spinal cord injuries or neurosyphilis, can disrupt normal hormonal regulation, potentially leading to nipple discharge.