Serotonin
From Tardis Wiki, the free Doctor Who reference
K9 noted that Sarah Jane Smith had elevated levels of serotonin when she was in love with Peter Dalton. He also reported high alpha waves and a faster heartbeat. (TV: The Wedding of Sarah Jane Smith)
According to Ianto Jones, chocolate was good for one's serotonin levels. He wasn't sure if Myfanwy had serotonin levels, though. (TV: Fragments)
During the Tenth Doctor's speech, the Dalek-humans displayed increased levels of serotonin. They subsequently questioned orders when commanded to kill the Doctor, and fought back together. (TV: Evolution of the Daleks)
