Hi ya
I have had an underactive thyroid since I was 12 years, and I have three children. Its just a case of making sure your medication is the right dose. It can change during pregnancy, but your doctor should keep on top of it.
I have only been diagnosed with diabetes for a few months so I no expert but I would think it would be the same, making sure meds are at the correct dose but eat as healthy as possible, just as any pregnancy.
Hope this helps, welcome to the forum its been invaluable to me.
Donna x