I’d second @"Kiwi Angel" suggestion - two reasons- one is anaemia and there are 2 types iron/folic acid deficiency.Plus I know a quite a few post menopausal women now on thyroxine as their thyroid gland has slowed (?) Personally I think I am still getting over several anaesthetics from surgery and radiotherapy despite being 16 months post diagnosis but like @primek has indicated exercise has helped a lot with my energy and sleep but needs to be done carefully so as not to overdo it.