Hi Kathleen, so sorry to hear of your diagnosis. Always a shock & time of uncertainty when we learn of a new condition, but just wanted to say most people do very well on Thyroxine, so you should start to feel much better in many ways once your optimal dose has been established. I remember your admirable approach to being diagnosed with UC & how well you've managed the condition, so I have no doubt you'll learn all you can about the thyroid, get yourself back to very good health and become an expert patient.
I was told many years ago by a well-loved & prominent Endocrinologist that the onset of my Hashimoto's & consequent hypothyroidism (diagnosed more than 25 years ago) were likely caused by my having adult onset Chicken Pox, that being one of two viruses he said commonly led to thyroid failure - the other being glandular fever, which perhaps adds weight to your intuition.
If it helps to chat to someone who's been around the medical block a good few times with this, please PM me any time.
Wx