Last time I checked, he suffered from hardening in his arteries which it seems became a factor behind his fluffs in the show (which was shown in An Adventure in Space and Time). However, Gerry Davis (who co-wrote The Tenth Planet and co-created the Cybermen who first appeared in it) once stated that The Tenth Planet wasn’t going to feature the 1st Doctor’s regeneration (which wasn’t named that back then) which was actually a last-minute rewrite to it. Davis also stated that Hartnell was never unwell then (only in the 1970s) and was actually fired for being difficult to work with (something which was also shown in An Adventure in Space and Time, possibly caused by his illness). So, was it a combination of those reasons or were we made to think that he was ill when he actually got fired?