As ever, the NICE Clinical Knowledge Summaries are very helpful! According to them:
Graves' disease causes hyperthyroidism-- your thyroid starts pumping out far too much thyroid hormone-- which among other things is likely to cause unintentional weight loss.
Carbimazole is a drug used to try to decrease the amount of thyroid hormone your thyroid produces, and this can sometimes succeed in putting Graves' disease into remission.
If this doesn't work-- you will be given radioactive iodine treatment. This kills off thyroid cells. "Most people with Graves' disease become euthyroid and then hypothyroid within six weeks to six months after completing radioiodine treatment." And then: "If the person develops hypothyroidism after treatment ... levothyroxine (LT4) replacement therapy should be started." (
https://cks.nice.org.uk/topics/hyperthyroidism/management/management/ )
So-- a lot of your thyroid cells were killed off in February, and that has made you hypothyroid-- and hypothyroidism causes weight gain. Your doctors will soon start giving you thyroid-hormone replacement therapy, and then you will get back to your normal weight.
I'm very concerned when you say you're so unhappy with the weight gain that you won't go out of the house! You should speak to your thyroid doctors as soon as possible, so they can get you started with the replacement therapy. I'm sure once you do, you will feel much much better. Wishing you all the best!