Is *th* ever used not as a thorn outside of portmanteaus?
Solution 1:
In the geographical names: Thames, Thame, Theale (although local opinion appears to be divided on that one), Thailand, Lesotho.
In the names "Thomas", "Thomson", "Esther".
The only common noun, i.e. not a proper noun, I can think of is "thyme".
Solution 2:
The river Thames, Thailand, thalweg (geographical term, although sometimes spelled talweg), Theresa are some.