What remarkable difference does Mr. Utterson notice about Dr. Lanyon?

On January 8, Mr. Utterson eats dinner with Dr. Lanyon and notices nothing out of the ordinary.  Six nights later, however, he returns to the doctor's house and meets a very changed man.  At this point, Dr. Lanyon

had his death-warrant written legibly upon his face.  The rosy man had grown pale; his flesh had fallen away; he was visibly balder and older; and yet it was not so much these tokens of a swift physical decay that arrested the lawyer's notice, as a look in the eye and quality of manner that seemed to testify to some deep-seated terror of the mind.

The formerly healthy doctor now looks to be on the edge of death.  He has deteriorated in almost every physical way in less than one week, and when Utterson asked about his altered appearance, the doctor resolutely states that he is "a doomed man." 

When pressed to explain, he only says that he's had a terrible shock and that he no longer wants to see or hear news of Dr. Jekyll, a man he now regards as "dead."  Utterson receives a letter from Dr. Lanyon with instructions not to open it unless or until Henry Jekyll has disappeared, and Dr. Lanyon himself perishes.  When Mr. Utterson eventually does open this letter, he learns that Dr. Lanyon actually witnessed the transformation of Edward Hyde into Henry Jekyll, and this was the shock that eventually lead to his dramatic physical change and premature demise.