Diabetes Insipidus vs. Diabetes Mellitus

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Diabetes is a term used to describe two different conditions. This lesson will describe the pathologies of diabetes insipidus and diabetes mellitus, explaining why they have similar names despite being different disorders.


You hear 'diabetes,' and you're probably fairly certain you have an idea of what someone is talking about: some disease where a person can't eat too much sugar. You'd be right . . . sort of. There are actually two separate diseases that are unrelated, but they both have 'diabetes' in their name: diabetes insipidus and diabetes mellitus.

If they're unrelated, why do they both have 'diabetes' in their name? Excellent question. The word diabetes was coined long ago to describe a condition where someone urinated frequently. Now, 'diabetes,' sure doesn't sound like 'peeing a lot,' but it originates from a Greek word that means to 'go through,' so when someone urinates excessively, a lot of urine 'goes through' him or her. And, while diabetes insipidus and diabetes mellitus aren't related, a symptom they both share is excessive urination.

Since their names are so similar, let's look at the second part of each name. Insipidus originates from a word in Latin that means 'no taste.' Contrast that with mellitus, which means 'honey' or 'sweet' in Latin. While this might not seem like a good clue, it actually tells you a lot. The urine in diabetes insipidus has 'no taste,' while the urine in diabetes mellitus is 'sweet,' which leads us into the pathologies of each disease.

Diabetes insipidus

Now don't go tasting urine to see if it is sweet or has no taste, but understanding the differences will help you see how these diseases work. Diabetes insipidus is caused by problems making, releasing, or responding to the antidiuretic hormone (or ADH) and results in excessive thirst and urination. So, while there is an excessive amount of urine, sugar will not be found in the urine (hence, the 'no taste' name of insipidus).

Let's get some background information under our belts so we can understand how ADH problems cause thirst and excessive urination. ADH is made in the hypothalamus and released by the pituitary gland. This hormone tells the kidneys if they should retain water in the bloodstream or if they should release water into the bladder. When there isn't enough ADH, the kidneys keep releasing water (and the person urinates a lot and is super thirsty since they aren't keeping enough water in their body).

Now sometimes the hypothalamus and pituitary get damaged. This can be due to brain tumors, brain lesions, head injuries, and even genetic factors. When this occurs, ADH isn't produced as it needs to be, which results in excessive thirst and urination. This type of diabetes insipidus is called central diabetes insipidus.

Note the location of the hypothalamus and pituitary in the brain
Brain diagram

Sometimes the kidneys don't work as well as they should and they don't respond well to the ADH. This is referred to as nephrogenic diabetes insipidus. Rarely, pregnant women get gestational diabetes insipidus, where the placenta, the organ that nourishes the unborn baby, destroys the mother's ADH. Finally, there is primary polydipsia where a person drinks too much water, which actually messes up the body's ability to make ADH. This can be the result of a metal illness or damage to the hypothalamus, which controls a person's thirst level. So now that you know a little more about diabetes insipidus, let's check out the other version of diabetes.

Diabetes mellitus

Diabetes mellitus is caused when there is too much sugar in the blood. When someone has diabetes mellitus, the extra sugar often comes out in the urine, which would make the urine 'sweet,' hence the name 'mellitus.'

So, why is there too much sugar the blood? Typically, the sugars you eat are broken down into glucose, and glucose feeds your cells. But it's not that simple. A hormone in your body called insulin tells your cells they can take glucose out of your bloodstream and use it. The problem with diabetes mellitus is that your body either doesn't make enough insulin or your cells don't respond to your insulin. Since your cells don't take up the glucose, it hangs out in your bloodstream and can actually damage your blood vessels. The body tries to get rid of the extra glucose by flushing it into the urine, which results in excessive urination (and glucose, or sugar, in the urine). Because the person is urinating so much, he or she will also be thirsty (just like diabetes insipidus!).

People with diabetes mellitus need to check their blood sugar
Blood glucose monitor

Like diabetes insipidus, there are different types of diabetes mellitus. Let's quickly go over those types, starting with type 1 diabetes, which usually manifests in childhood. In this form, the body's immune system attacks the pancreas, preventing it from making insulin. Type 2 diabetes usually develops later in life compared to type 1, and can result from obesity. Oftentimes in type 2 diabetes, a person's cells are resistant to the insulin, so even though the pancreas makes insulin, it isn't enough. Gestational diabetes can occur in pregnant women because pregnancy can lead to the cells resisting insulin. Gestational diabetes goes away after the pregnancy is over.

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