One cause of a lump in the wrist is arthritis at the base of the thumb, explains UConn Health. Moderate to severe pain and weakness in the grip also accompanies hand and wrist arthritis. Another common cause of a lump in the wrist below the thumb is a ganglion cyst, according to WebMD. It is uncertain what causes ganglion cysts, but theories include trauma to the area or a flaw in the joint capsule that allows joint tissue to escape.
Ganglion cysts may occur on both the top and bottom of the wrist, at the base of the fingers in the palm or on the outside of the knee or ankle, notes WebMD. They are usually soft, may change in size and sometimes cause pain.
A medical evaluation of a ganglion cyst includes drawing out the fluid and an ultrasound, which reveals whether the cyst is solid or fluid-filled and if it involves any blood vessels, reports WebMD. Medical treatment involves aspiration, injection of a steroid compound and splinting the wrist to keep it stable. Surgery is often effective for cysts that continue to return. Home remedies include poultices, topical plaster and heat, but they are generally ineffective at removing the cyst permanently.