Investigators, reflecting upon his sickly nature and having a knowledge of medicine during those times, exhumed the remains of Ivan, buried at the Kelmisvi Chapel, Moscow. Forensic tests on those remains conclusively point to a high level of mercury. Mercury may—or may not—have been the cause of Ivan’s death. There are those who feel he died from syphilis. Even if that is the case, the treatment of choice for syphilis was often—you guessed it—mercury.

http://www.quirkyscience.com/what-drove-ivan-the-terrible-mad/


http://www.telegraph.co.uk/news/worldnews/europe/russia/1326387/Mercury-poisoned-Ivan-the-Terribles-mother-and-wife.html