Why is "Theresa" pronounced with the plosive /t/?
Judging from the spelling I always thought Theresa was pronounced with an interdental fricative. On the German news I often heard it pronounced with a /t/ as initial consonant. I thought this was due to the fact that German does not have interdental fricatives which often results in Germans replacings those with plosives (/t/ or /d/) or sometimes with the dental fricatives /s/ or /z/. The other day, however, I watched BBC World News and heard the prime minister's name Theresa May being pronounced with the plosive /t/. On the German Wikipedia page for Theresa May the IPA pronunciation is also given with the plosive sound.
Is there any reason for this pronunciation and, if so, are there other exceptions where the th-spelling is pronounced with a plosive instead of with the usual interdental fricatives?
Solution 1:
There are a number of other words or names where "th" corresponds to /t/. Uwe listed some in a comment: Thomas, Thames, Thailand, thyme.
The digraph "th" was originally used in Latin to transliterate Greek θ. But in the languages that descended from Latin, "th" became identical in sound to "t". This led to some interchange of "t" and "th" as spellings for /t/ in languages that descended from Latin, such as French, or that were otherwise influenced by Latin spelling, such as German.
The "th" digraph came to be used in English for the native sound /θ/ (and its voiced counterpart /ð/). But English also has many loanwords from Latin, French or even German that had "th" = /t/. These have been treated in different ways.
In some cases, English retains the /t/, as in "Theresa" (corresponding to Spanish "Teresa" or French "Thérèse", both with /t/).
In other cases, it has been replaced with /θ/ due to spelling-pronunciation, as in "author" from a variant spelling of French autor (from Latin auctor).
In a few cases, the sound represented by "th" is variable and can be /t/ or /θ/ depending on the speaker or on the person bearing the name. The word Neanderthal, from German, is etymologically just a spelling variant of Neandertal, so some people pronounce it with /t/. But a spelling-pronunciation with /θ/ is more common. The name Anthony, from Latin Antonius, is traditionally pronounced with /t/ in British English, but often pronounced with /θ/ in American English. The usual modern attitude is that the "correct" pronunciation of a name is the pronunciation used by the specific person being referenced (with some allowances for differences in phonetic inventory, phonotactics and allophony between accents).
Solution 2:
The name is actually Teresa. Theresa is just a variant spelling that somehow kept the original pronunciation.
https://en.wikipedia.org/wiki/Teresa