Serotonin

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, as a pterodactyl. (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)