ADTH was a stimulant which could be dispersed through a starship's life support systems.
In 2367, Lieutenant Commander Data put five parts per million of ADTH into the airflow of the USS Enterprise-D to wake the crew after they had been stunned by the Paxans' fake wormhole. (TNG: "Clues")
The name is likely a reference to ACTH, Adrenocorticotropic hormone. ACTH is released by the pituitary gland, it stimulates the cortex of the adrenal gland and is also helps regulate circadian rhythms.