Uma boa questão:
Etymologically, th in English corresponds to the similar sounds in Old Norse and Ancient Greek, but while Norse had both the voiced and voiceless forms, as far as I’m aware Greek had (and has) just the voiceless sound — represented in writing by the letter theta (θ). To paraphrase the OED, the Romans had neither the sound nor the symbol, and so represented the letter by th, but apparently this was pronounced, at least in late Latin (whence all the Romance languages) as a simple t. The OED gives the example of the Greek word θεωρία, which in Latin is theoria, Italian and Spanish teoria, Portuguese theoria and French théorie (the latter spelt with th and pronounced with t).
A handful of examples in English, by the way, where th is pronounced as t, are the words Thomas, Thames and thyme. There are also words where in writing t is followed by h, but in a different syllable. In these, the t and the h are pronounced separately, as in lighthouse and anthill."
Em resumo, pouquíssimas palavras no inglês moderno: Thomas, Thompson, the Thames, thyme.