Sorry you feel rotten. Its bad enough worrying about diabetes without feeling under par with no explanation
Re tests- consider getting them to check iron/b12 levels along with thyroid. Maybe also urine, check whether there is low grade infection showing up that might be asymptomatic? I've not felt flu-y but have similar probs and consultant says they tend to check for other autoimmune diseases in people with dx.
If GP thinks you are viral - dont know why you are getting antibiotics - they wont work on a virus.
Hope you feel better soon.
Lilibet