What causes terrible pain on top of hand between thumb and index finger?

Tendonitis: Possibly. There is a common tendonitis called dequervanes tendonitis that can cause sharp pain at the base of the hand, behind the thumb, sometimes radiating into the thumb/index interspace. Is the pain worse when you make a fist but enclose your thumb within your fingers and squeeze? Past that first thought, would nee more info, such as trauma or other symptoms to have any other suggestions.