Large deposits of Kironide were present on the planet Platonius. When the Platonians, originally from Sahndara, colonized the world, they gained extraordinary psychokinetic powers, which they used to entertain themselves by sadistically torturing Alexander, a Platonian who did not have the powers. It was later on discovered that they had gained these powers due to the kironide present in the local vegetation they ate, and that Alexander did not have the powers because he possessed a pituitary deficiency which prevented him from metabolizing the kironide.
In 2268, the starship USS Enterprise arrived on the planet. For the landing party to escape 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.