it's very swings and roundabouts, though, Rosella
HT is more likely in older cats, but then so are other conditions, especially CRF, which would tend to rule out an irreversible cure like I131
I think with older cats, say over 14, their ability to tolerate medication is perhaps the most important factor - some cats cannot tolerate Felimazole or Vidalta, and for them only surgery or I131 are options - and for the older cat surgery might well be considered the more risky procedure, as I131 in itself doesn't seem to pose any health risk, as long as the kidneys don't crash
with a cat Trigger's age, medication for perhaps 7+ years, while the thyroid tumour continues to grow, is not IMO a happy prospect