Does the Mad Hatter have mercury poisoning?

Many hatters of the time suffered from Mercury poisoning, and this is almost certainly the source of the phrase "as mad as a hatter", as many of the effects of this toxin could appear to be symptoms of 'madness'.

However, the Mad Hatter probably does not have mercury poisoning.

The symptoms of mercury poisoning include such psychological disturbances as: irritability, fits of anger, lack of attention, low self confidence, drowsiness, and decline of intellect.

While the Hatter arguably suffers from a lack of attention when, in Alice's Adventures in Wonderland, he makes the swift subject change from Alice's hair to ravens and writing desks, he displays none of the other psychological symptoms.

Neither is any mention made of him showing any of the physical symptoms, which include: foul breath, dizziness, ringing or noises in the ears, fine tremors, coughing, and loss of appetite - any or all of which Carroll could have included in his depiction of this character. (To see more about the symptoms of mercury poisoning, follow the Related Link below)

It was probably more the case that the character was based on the saying rather than on the illness that caused the saying to be coined.

And, as Martin Gardner points out is his Annotated Alice, it is generally considered that the Hatter's character was inspired by "Theophilus Carter, a furniture dealer near Oxford. Carter was known in the area as the Mad Hatter, partly because he always wore a top hat and partly because of his eccentric ideas." (From The Annotated Alice by Martin Gardner)