What can cause the joint that connects my thumb to my hand to hurt?

Arthritis: The joint at the base of the thumb is a common location for osteoarthritis. Pain is typically worse with thumb use, particularly pinching, opening jars, and turning keys. Initial treatment is anti-inflammatory medication, rest, and use of a thumb brace. If pain persists you should see a hand surgeon.

Thumb CMC arthritis?: One of the most common causes of pain at the base of the thumb is arthritis. Pain usually increases with pinching or opposing the thumb across the palm. An x-ray may help with diagnosis. Maybe try a splint that immobilizes the thumb for a while to see if it helps. Good luck!