Thomas Addison (April 1793 – 29 June 1860) was a renowned 19th-century English physician and scientist. He is traditionally regarded as one of the "great men" of Guy's Hospital in London.Among other pathologies he discovered Addison's disease (a degenerative disease of the adrenal glands) and Addisonian anemia (pernicious anemia), a hematological disorder later found to be caused by failure to absorb vitamin B12.