It is derivative of Euthymia, the Greek noun to "sweet natured".

Martyrs Chrysanthus and Euthymia.

Hyperthymia can be most accurately diagnosed by a psychiatrist with the help of a patient's family and/or close friends, as these individuals are most likely to have noticed a change in the individual's temperament from euthymia.

But still he does not appear to have felt very safe in his position, as we may conjecture from the extreme delight with which he received the news of Alexander's death, in consequence of which he erected a statue of euthymia, that is, joy or peace of mind.