This is gross.
Unless you're a medical professional, please don't armchair diagnose. If you are, I'm pretty sure you were taught not to diagnose mental illness of celebrities without meeting them, as all medical folks I know personally have told me they were taught.
That is indeed what is taught, but first up, some things are just so obvious that you don't actually have to examine anyone to "diagnose" correctly. Second, we are talking about the most powerful man on earth, so this is not just meanspirited gossiping, this is actually important and a vital interest to the public.
For the record I am a psychologist, although for the last decade and a half I have taught psychology and political science instead of practising psychology.
Valid points in regards to the circumstances. That said, I'm still uncomfortable with non-professionals trying to diagnose, as per others in this thread. In addition, it's still obvious that Trump is unfit without bringing possible mental illness into the picture. We don't need a condition to prove his fitness or not, and assuming an illness would be enough to make him unfit is also problematic. So, by all means, let's discuss his unfitness, but let's leave possible illnesses out of it. The point is that he isn't able to appropriately do the job, and that is obvious without any other information.