Hurt thumb wrestling at my job (le) now it hurts at the base of thumb when i extend it and attempt to write what could be wrong?

Needs assessment: Variety of possibilities and will need an assessment by a doctor licensed in your area. 2 important injuries to exclude rapidly are a bennets fracture (the tendon avulses/rips off bone at the base of the thumb) and a collateral ligament injury. Both if diagnosed and treated promptly can have much better chance of healing well. Please see a doctor for examination and to consider X-rays

See doctor : There is possibility of fracture or ligament injury . Would need examination, please See your doctor.

Need splint: It may be either a bone or ligament injury (fracture or sprain). Since work related, should report and get it checked by Work Comp Dr. Meanwhile, support, tape, or splint to decrease painful movement + cold compress to area. If not gun-hand, can be FD w/sprained thumb. Get it looked at & be safe!