In Gregg shorthand (simplified), “thorough” is written th-e-r-o.
I would have used different vowels there, so I tried to see where those came from.
The first was easiest; I was expecting a STRUT vowel there, since I have STRUT in case such as “hurry”, but I have heard NURSE in such words from Americans. Essentially, I have “hu-ry” while they have “hur-y”. (I do have NURSE in words where I segment things with the r in the same syllable as the u; for example, in “furry”.)
OK, so this presumably represents a pronunciation with NURSE; that sound is regularly spelled e-r, so that made sense.
But I have commA at the end of the word; the vowel in both syllables is nearly the same for me. So seeing an o there seemed odd. (So I would have spelled the word th-oo-r-a, since oo is used for STRUT.)
I checked dictionary.com and while that gave both STRUT and NURSE for the first syllable, it gave only GOAT for the final vowel.
Then I had a look at Forvo; that had seven pronunciations recorded. Clicking through them one by one, it seems that there is a Commonwealth/US split for this word, with commA for the former (the UK samples and the Australian one) and a fairly clear, unreduced GOAT for the latter.
Huh! Learn something new every day.
(And now, thorough sounds extremely odd to me. Typical result of listening to a word over and over!)