Went from HyperThyroid to HypoThyroid?

I went from 112 lbs. and found that I had Hyper Thyroid. I took the Radiation/Iodine pill to stabilize thyroid...Now, I have gained 12lbs. in 2 weeks, and have no energy, I thought I would gain 5 lbs. and still have energy. Has this ever happen to anyone? And why am I so Lethargic? Please give an answer..Thanks, I will be waiting!