But a thorn simply changed to a th, and was pronounced the same, no? The pronunciations for Þ are [θ] and [ð], which are spelled as "th" in modern English. ([θ]

But a thorn simply changed to a th, and was pronounced the same, no? The pronunciations for Þ are [θ] and [ð], which are spelled as "th" in modern English. ([θ] is "th" as in "think" and [ð] is "th" as in "father). So I'm unsure what you mean by "My 'Th' comes from 'thorn', not 'th'". The digraph "th" simply underwent a shift in Old English to spelling it with Þ, and then shifted back to "th". (On a related note, though, it's always funny when "Ye olde shoppe" is pronounced differently that "the old shop." The "y" is simply a version of the thorn Þ, which was pronounced "th", and the ending "e"s were not pronounced.)