Varicose veins: This is the most common cause of swollen veins on the hands, often due to increased pressure or weakened valves within the veins, leading to their enlargement.

Thrombophlebitis: Inflammation of the veins due to a blood clot, which can cause swelling, pain, and warmth in the affected area.
Venous insufficiency: A condition where the veins have problems returning blood to the heart, leading to swelling and enlarged veins.
Dehydration or volume depletion: Reduced fluid volume in the body can cause a decrease in blood volume, leading to prominent veins.

Deep vein thrombosis (DVT): A blood clot in a deep vein, which can be life-threatening if it breaks loose and travels to the lungs (pulmonary embolism).
Superficial thrombophlebitis due to malignancy: In rare cases, thrombophlebitis can be associated with underlying malignancies, such as pancreatic cancer (Trousseau’s syndrome).
Infection or sepsis: Although less common, swollen veins can be a sign of an underlying infection, especially if accompanied by other symptoms like fever or redness.

Klippel-Trenaunay syndrome: A rare congenital condition characterized by abnormal development of blood vessels, leading to varicose veins, among other symptoms.
Parkes Weber syndrome: Another rare condition involving abnormal blood vessel formation, which can cause varicose veins and other vascular anomalies.
Sarcoidosis: A disease that can affect multiple parts of the body, including the skin and veins, though it's a less common cause of swollen veins on the hands.