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Other Types of Diabetes

Most people have Type 2 diabetes

Most people with diabetes - about 95 to 99% - have Type 1 or Type 2. But not everyone fits neatly into these two groups. Some people have less common types of diabetes, which are important to recognise.

Of those not clearly Type 1 or Type 2, many belong to a mixed group of persons with diabetes that include many smaller categories; here I will list a few of these categories and briefly explain them.

What are some of the rare forms of diabetes mellitus?

Maturity Onset Diabetes of the Young (MODY): this is a group of typically one-gene disorders that result in the person and their affected family members developing diabetes at a younger-than-expected age. The severity of the diabetes can range wildly from mild to severe - very interestingly, there is one mutation (MODY 2, GCK mutation) in which there are almost no ill-effects of a higher than normal baseline blood sugar level even after many years. You would need a genetic test to confirm this type of diabetes.
Pancreatogenic diabetes (previously known as Type 3c diabetes): this diabetes arises from destruction of the pancreas outside of the auto-immune mechanism in Type 1 diabetes. The resultant illness typically also results in issues with insulin's opposite hormone, glucagon, and exocrine enzymes that help digest your meals. The disease usually follows a course of typically severe illness like pancreatitis.
Secondary diabetes: This is a group of diabetes usually caused by another illness or medications taken; the most prominent example here is that of steroid-induced diabetes (used here to refer commonly to glucocorticoids) . Glucocorticoids (like prednisone) are well known to raise the blood sugar level, and if taken for a longer period of time, can result in permanent diabetes (among other issues).
Wolfram syndrome/DIDMOAD: This is an example of a genetic condition that presents with a constellation of symptoms - in DIDMOAD, the acronym stands for Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy and Deafness, which is why sometimes your endocrinologist seems very interested in your hearing aids!
Mitochondrial diabetes: This is an even rarer diabetes associated group where the primary issue lies inside the mitochondria. The clue in the history is one of diabetes almost exclusively inherited in by females and their children - this happens because the mitochondria are passed down from mother to child and are implicated in the diabetes in this group.

There are many other rare forms of diabetes mellitus and this list does not cover them all.

It is important to note here that endocrinologists also deal with a separate endocrine disorder called Diabetes Insipidus. It is important to know that diabetes insipidus has nothing to do with glucose but a problem with a water-regulation hormone called vasopressin.

Should I get tested for genetic diabetes?

Type 2 diabetes, the most common form of diabetes, has a strong family/genetic basis. In fact, this genetic basis is typically "stronger" than those with Type 1 diabetes.

Because of this, we do not typically rely on a family history of diabetes to tell us about the risk of rare diabetes - what we look for is an unusual feature in the family history where, for example, everyone with diabetes is diagnosed at a young age, or the diabetes progresses rapidly through tablet therapy into insulin therapy for all members. We also look for associated features in the family like a high rate of unexplained deafness in the family or muscular weakness that results in disability.

If you think you might have a genetic basis for your diabetes in your family, the best thing to do would be to draw a family tree and identify who in the tree:

has diabetes
the age of insulin start, if on insulin
has unusual early onset features like deafness or other illnesses
has complications of diabetes and at what age

If you do not have this history or are unable to get it, still raise the question if it worries you; most of the time, your endocrinologist would be able to provide some guidance around what they think your risk of rare diabetes might be and offer access to genetic testing if indicated.

Some rarer forms of diabetes also have pre-testing risk calculators. Specifically, the MODY risk calculator by Exeter is used broadly by many endocrinologists to assess a person's risk of having MODY, which can then be tested using genetic testing.