The thyroid was a large gland found in a humanoid's endocrine system, which regulated hormones in the body.
In 2267, to snap Spock out of his Omicron spore-induced spell, James T. Kirk taunted him by calling him, among other things, "an elf with a hyperactive thyroid" (TOS: "This Side of Paradise")
In 2371, Doctor Julian Bashir found no thyroid problems while examining a Jem'Hadar baby Quark had unknowingly taken possession of. (DS9: "The Abandoned")
External link Edit