Thad and Thelma
The th digraph has a voiced and unvoiced sound. The voiced th produces vibrations when pronounced, such as in these. The unvoiced th produces no vibrations when pronounced, such as in Thad. There are a few instances in which the th digraph is pronounced differently, as in Thailand and thyme, but exceptions are few and far between. There are extremely rare exceptions in which another group of letters is used to represent the /th/ sound.
Note: There is a subtle difference between the pronunciation of the voiced th in there and the voiceless th in thing.