Does blood become thinner at high altitudes?

It would seem so. Much below 12,000ft, I've not personally observed a significant difference. However, in higher altitudes, there are definite if anecdotal clinical observations of pulmonary edema, cerebral edema, gastrointestinal bleeds and various other similar phenomena. I've linked a study conducted in Tibet with high-altitude railroad workers, who spent extensive time at altitude and yes -- significantly more GI bleeds, although the article doesn't indicate the number of fatalities, if any, attributed to this.