Language / Need help with spelling "Theresa" [4]

Hi
I am new to this forum.
I've been looking on the internet for the correct spelling for the nickname for the english name Theresa. I only know how to pronounce the nickname, and I always thought is was Polish but last night my mom thought it was French Canadian. (She also thought Kokopelli was italian LOL; it's american indian) I think its Polish because Theresa was a relative on our Polish side of the family.

Anyway, we always pronounced it "Tuh-di". "Tuh" rhymes with "duh" and "di" rhymes with "lie".

My spelling guesses: Thudai, Thedi, Tudai
Any help would be much appreciated!
Suzanna