John Sotos is a physician well known in medical circles for his book, ZEBRA CARDS: AN AID TO OBSCURE DIAGNOSIS. We tell our medical students, "When you hear hoofbeats think horses, not zebras," or, common things occur commonly. But John has had a lifelong interest in rare diseases--zebras. (He is also a medical consultant for the TV show HOUSE.)
Recently, I heard John make the case in a lecture that Abraham Lincoln suffered from a rare endocrine disorder called MEN2b (Multiple Endocrine Neoplasia Type 2b) in which most patients will die of cancer of the thyroid if the thyroid is not removed. John argues that at the time of his death, Lincoln was dying. Indeed, if one looks at Lincolns pictures
from January 1864 
and then a few months later in February 1865,
it does appear that he has lost weight and reports from that time suggest he was greatly fatigued. One characteristic of this syndrome is the presence of mucosal neuromas--little bumps on the lips. Close examination of photographs such as the one below suggests that he did have these.
The Grand Army of the Republic museum in Philadelphia has a pillowcase with Lincoln's blood on it, and DNA from that could conceivably prove or disprove this hypothesis. (It could also prove or disprove another hypothesis that states Lincoln had another condition, namely Marfan's syndrome.) For now, the museum has decided to wait on DNA testing.
