Hyperthyroidism was a disease that came as a result of a hyperactive thyroid, which primarily increased metabolism, but had a vast amount of varying physical effects.
When James T. Kirk discovered, in 2267, that violent emotions were the means required to destroy the Omicron spores that had taken over his crew, he was forced to make several insulting remarks to his first officer Spock in order to break him free of the spore's influence, including a reference to him being "an elf with a hyperactive thyroid," an obvious analogy to his Vulcan ears and physique. (TOS: "This Side of Paradise")
When an unidentified alien youth was discovered by Quark and taken to the Infirmary to be studied by Doctor Julian Bashir, in 2371, Dr. Bashir noted that the youth had a "a very fast metabolic rate," and that "his cells are dividing at a pace I haven't seen outside of a laboratory." Bashir was able to determine that his patient's abnormal growth was not due to "thyroid or hypothalamic problems," and that such rapid growth "may be natural for his species." It was a short time later that Bashir was able to determine that this 'natural hyperthyroidism' came as a result of Dominion genetic engineering, and that his patient was actually a young Jem'Hadar. (DS9: "The Abandoned")