What is the difference between diabetes mellitus and diabetes insipidus?

DThe word diabetes is an interesting one. Its origin is in the  Greek language where it is derived from the word for a  siphon  or, more simply, a pipe or hose. This  word  was  used   to  describe  the  disorder  in ancient times (and diabetes was  recognized in great antiquity) because those suffering from it  produced such plentiful amounts of urine that they were remi- niscent of a water pipe. The reason for the plentiful amounts of urine lies in the fact that when the sugar glucose reaches  excessively high levels in our blood- stream, it is filtered into  the kidney and enters the urine  in  large quantities.  Due  to  its  chemical and physical properties, when large amounts of glucose are filtered by our kidneys into the urine, it cannot be fully reabsorbed and retains a large amount of water with it, thus creating very large volumes of urine. The second part of the name, mellitus, is  derived from the word meaning sweet, as in mellifluous music.  Mellitus was added when it was discovered that the urine in a person with diabetes and very high blood sugar is sweet.

Diabetes insipidus is a disorder with an entirely dif- ferent  basis, but  its  sufferers share the  siphon-like quality of very frequent and very high volume urina- tion. Diabetes insipidus  is due to failure of produc- tion  or action of another  vital  hormone, known as arginine vasopressin (AVP), also called antidiuretic hormone (ADH), that is responsible for maintaining the normal volume and concentration of our  urine. When AVP is deficient (usually due to damage or dis- ease of the hypothalamus or pituitary gland) or fails to work (usually due to disease of or damage to the kidney), we are unable to concentrate our urine and it becomes excessively dilute. As such, it appears pale, almost col- orless and watery—in a word insipid, hence insipidus. It is not sweet, as it has negligible amounts of sugar in it.