I am mildly diabetic and have thyroiditis. I often have itching & rashes at night. Could out be due to these diseases?

Possible. There are different skin manifestations related to diabetes and sometimes medicines for treatment of both can cause rash so have your doctor reveiw your medicines to see if one is the culprit and examine you for rashes to see if related to these diseases.
Yes. Rashes and itching can be associated with diabetes including peripheral' nerve damage, color change on the lower legs or even blisters on the lower legs. Prewcription topical/, steroids and topicalxylocaibe or zonalon (doxepin) can offer relief. The diabetes'must be well controlled. For peripheral nerve changes Alpha lipoid acid may be helpful.
Have it evaluated. There are many things that could cause the skin to itch. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Insect bites can also be the cause of a rash on the skin. Have it evaluated by a professional and get the appropriate treatment.