While type 1 and type 2 diabetes are well-defined, the way in which less-common forms of diabetes are classified has changed over the years.

There is no single definition of type 3 diabetes.

Currently, the American Diabetes Association sets out four different groups of diabetes:

Note that type 1 and type 2 are the only forms of diabetes officially assigned a numbered type.

Historical classifications of diabetes types

In 1999, the World Health Organisation (WHO) published a report of a WHO consultation. The report, titled ‘Definitio, Diagnosis and Classification of Diabetes Mellitus and its Complications’, laid out the following areas of diabetes.

7.3.1 Genetic defects of beta-cell function

7.3.2 Genetic defects in insulin action

7.3.3 Diseases of the exocrine pancreas

7.3.4 Endocrinopathies

7.3.5 Drug- or chemical-induced diabetes

7.3.6 Infections

7.3.7 Uncommon but specific forms of immune-mediated diabetes mellitus

7.3.8 Other genetic syndromes sometimes associated with diabetes

These categories have led to some researchers and healthcare professionals to name these forms (7.3.1 to 7.3.8) as type 3a through to type 3h diabetes. However, these numbered forms have not been officially recognised by major diabetes organisations such as the World Health Organisation or the American Diabetes Association.

In research literature, the term type 3c has been used for pancreatogenic diabetes (which includes diabetes that may result from pancreatitis), which falls under 7.3.3 (diseases of the exocrine pancreas) in the above list.

It is useful to be aware that the use of ‘type 3’ terminology sometimes appears in common usage but is not a term that is officially recognised.

Other unofficial uses of type 3 diabetes

In 2008, researchers from Brown University, Dr Suzanne de la Monte and Dr Jack Wands, put forward a proposal that Alzheimer’s disease could be termed type 3 diabetes.

The reasoning is based on the fact that insulin resistance within the brain was shown to be a feature of Alzheimer’s disease.

While this has been proposed, major health organisations do not recognise Alzheimer’s disease as a type of diabetes within their classifications.