I could have posted your original message! I just finished my RAI last week, and started my Synthroid last Friday (150mcg).
I also feel very lathargic, puffy, and my weight is creeping up despite not eating a lot. I had quite a headache from the RAI, and it persisted for a few days once I got home. Also, my neck was painful along the sides and at the back of my head. I was told that was normal, or could be referred pain from the thyroid bed area.
My whole body scan on Monday didn't show any distant mestasis, and nothing outside the thyroid bed lit up, so I am grateful for that good news. BUT, I have to wonder when I'll begin feeling better? I was told it may take 6 weeks for the meds to reach their peak. In the meantime, I feel like I could be used as a float in the Macy's Parade.