I bet telling people you're a phthisiotherapist at parties is a real conversation stopper. That's not physiotherapist but phthisiotherapist, beginning with phth. It's not a common word, but it means an expert in the treatment of tuberculosis. Phthisis, or phthisic, is another word for tuberculosis. The spelling phth in these words is a direct translation from the first two Greek letters, φ (phi) and θ (theta), of the word for 'wasting' or 'consumption'.
There are over 350 entries in the OED that contain the sequence of letters phth and most of them are scientific. The Dictionary says that there are various pronunciations of the word. In British English you can pronounce both letters, so that it sounds like fth, or just th. In American English these words are usually pronounced with a th sound alone, or the whole group phth is pronounced as a t (so a Brit might say 'fthizic' for phthisic but an American is more likely to say 'tizic').
When the phth comes in the middle of a word, you never pronounce a t sound at all. Ophthalmology is either pronounced 'offthal ...' or 'opthal ...', and many people misspell the word by missing out the first h because the ph is so often pronounced as a p.
The phth of the words diphthong and triphthong is usually pronounced fth, although the f sound is often missed out of monophthong because it's hard to say. Here phthong is the Greek word for sound.
For more on diphthongs and triphthongs in English see this old post of mine.