I'd mention it. Personally, if I saw an application like that with a rapid improvement in achievement and a diagnosis of a disorder like hypothyroidism that could easily account for it with initiation of treatment right about the time the improvement started, I'd all but disregard what happened before as not indicative of what the person can do now that they are stabilized.
Obviously that's just me and I'm not a PD, but I think it's obvious that for some percentage of programs that otherwise considered you borderline, this could easily be a game changer.
As was mentioned previously, you might want to talk to your dean and see if this can be included in your MSPE. They might say no (not sure why they would exactly), but if they include it, it might be a lower key and more "institutionally backed" acknowledgment of the reason for the shift in achievement.