In 2268, the starship USS Enterprise arrived at the planet known as Platonius, where the inhabitants had gained extraordinary psychokinetic powers, which they used to entertain themselves by sadistically torturing Alexander, a Platonian who did not have the powers. It was discovered that they had gained their powers due to the kironide that was present in the local vegetation, and that Alexander did not have the powers because he possessed a pituitary deficiency which prevented him from metabolizing the kironide.
In order to escape the control of the Platonians, Doctor McCoy injected Captain Kirk and Commander Spock with a high dose of kironide in order to reproduce the Platonians' powers. Their plan worked and they were soon able to overpower Parmen, the Platonian leader, and gain their and Alexander's freedom. (TOS: "Plato's Stepchildren")
According to the Star Fleet Medical Reference Manual, (pages 18, 35) kironide was an element, symbol Ki. It had the atomic weight 345 and atomic number 139. It was first discovered on Vulcan. When ingested, pituitary hormones brake down the kironide compound. While kironide compounds have been researched by Starfleet Medical, they have been found to have no real clinical use, as the hormones have a half-life of only 10 to 30 minutes in the blood stream.