Hi Feather, I'm so sorry to hear your poor Mum's QOL has suffered for her Thyroxine dose being lowered. It is said we need less as we age, as Stellajane mentioned. I don't know whether your Mum has other medical conditions that might lead her GP to be cautious & reduce her dose, but either way I think I'd want to explain to him that I felt worse for his action if I were your Mum. But I do understand her reluctance to question his decision - my Mum was the same in terms of reverence for her doctors. A difficult position for you when you are understandably concerned for your Mum's welfare but the GP's attitude also seems reasonable to you. I don't know whether confidentiality issues might prevent him going into more details with you about his decision, but maybe see if you can get him to tell you why what he's done is necessary?
Wx