Does ADHD and hypothyroid disease have similar symptoms making it possible to diagnose one instead of the other?

After all else. After all other diagnoses are excluded you can diagnose adhd. This means you must demonstrate normal thyroid levels for at least a month before you can accept an adhd diagnosis.
No, but hyperthyroid. Ism can mimic signs of adhd, as can behavior related to sleep apnea, vision & hearing deficits, some symptoms of autism, anxiety, ptsd &/or depression, situational adjustment reaction to sociofamilial stresses, thorough evaluation includes parent & teacher questionnaires & scored checklists, school records, psychological evaluations, growth, past medical & family history.