I recently received an e-mail inquiring about "type 3 diabetes." The writer stated "It's very confusing! People are now mistaking type 1.5 and type 3, even I had a hard time gleaning whether type 3 is type 1.5 and Alzheimer's linked, is it type 1 and type 2 with a twist? "
Yes, it's confusing. As Wikipedia points out, "Various sources have defined 'type 3 diabetes' as, among others, gestational diabetes, insulin-resistant type 1 diabetes (or 'double diabetes'), type 2 diabetes which has progressed to require injected insulin, and latent autoimmune diabetes of adults (or LADA or 'type 1.5' diabetes.)"
There was a flurry of press releases a few years ago, describing a link of Alzheimer's disease to diabetes and proposing the existence of a third type of diabetes. But that was only their opinion, and has not been accepted by the diabetes community.
It's clear that the standard "types" of diabetes (type 1 and type 2) don't cover all the possibilities for categorizing diabetes. But those who dream up classifications haven't included either type 1.5 nor type 3 as of yet, so everyone should be aware that there are no "official" definitions for 1.5 and 3.
The present official categories are discussed at the ADA website in a lengthy discussion, Diagnosis and Classification of Diabetes Mellitus, and include:
* Type 1 diabetes (T1DM), with absolute deficiency of insulin secretion. Most cases of T1DM are clearly autoimmune in nature, with patients usually having positive antibodies. The disorder typically shows up in children or teenagers. It previously was called insulin-dependent diabetes (IDDM) or childhood-onset diabetes. T1DM also includes a subcategory called "idiopathic diabetes" (diabetes of unknown cause). According to the official classification, this is a variant of T1DM, but without evidence of autoimmunity. It is more likely to occur in people of African or Asian ancestry. As I've written about previously, there has been casual use of the term "type 1.5" diabetes (usually pronounced "type 1-and-a-half diabetes") to describe a diabetes variant called Latent Autoimmune Diabetes in Adults (LADA). LADA is actually a form of T1DM, occurring in an older age group. The ADA's classification scheme clearly includes LADA as part of what they call T1DM.
* Type 2 diabetes (T2DM), with a combination of resistance to insulin action and an inadequate compensatory insulin secretory response. T2DM typically shows up in obese adults, whose antibody tests are negative. It previously was called non-insulin-dependent diabetes (NIDDM) or adult-onset diabetes.
* Gestational diabetes (GDM), defined as diabetes with onset or first recognition during pregnancy.
* Other specific types of diabetes. This includes several subgroups:
Genetic defects of the beta-cell. Several such disorders have been identified, including several variants of " maturity-onset diabetes of the young " (MODY) diabetes.
Genetic defects in insulin action. Again, several disorders have been identified.
Diseases of the exocrine pancreas. If the pancreas is damaged or destroyed, such as by cancer, injury, or infection, the beta cells of the pancreas that make insulin may be damaged or destroyed.
Endocrinopathies. If there is a disorder causing excessive secretion of hormones such as thyroid or growth hormone, diabetes may develop. Typically, the diabetes resolves when the other hormone problem is fixed.
Drug- or chemical-induced diabetes. The most common prescription medication that can cause diabetes is the class of drugs called steroids.
Infections. Some viruses have been associated with beta-cell destruction and subsequent diabetes.
Uncommon forms of immune-mediated diabetes. Other autoimmune disorders may be associated with diabetes, including the "stiff-man syndrome" and lupus (SLE).
Other genetic syndromes sometimes associated with diabetes. People with other chromosomal disorders, including Down's syndrome, Turner's syndrome, and Kleinfelter's syndrome, may have diabetes.
One might imagine that the folks who dreamed up this official classification might have continued beyond the two "types", so that (for instance) they might have called GDM by the label "type 3 diabetes." But they didn't; there are only two "types" and a whole bunch of other disorders that didn't get a number of their own.
In my opinion, anyone using unofficial terminology, such as type 1.5 or type 3, should acknowledge that it's unofficial, and clearly define what they are talking about. Otherwise, the confusion will continue.