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Part A. Introduction to Diabetes


The majority of cases of diabetes mellitus falls into one of two categories: (1) type 1 diabetes, which is most commonly caused by autoimmune destruction of the pancreatic islet cells resulting in β-cell failure and an absolute deficiency of insulin (type 1 A diabetes, T1DM), and (2) type 2 diabetes (T2DM), which results from a combination of insulin resistance in target tissues and a lack of an adequate compensatory insulin response to overcome this resistance. The American Diabetes Association (ADA)1 recognizes more than 50 other specific types of diabetes (Table 11-1). All of these disorders collectively account for only 1% to 5% of cases of diabetes (see Part D of this chapter later on).

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Table 11-1.   Classification of Diabetes Mellitus 

Ensuring a correct diagnosis and defining the correct type of diabetes is important for choosing the most appropriate therapy, managing the expected complications, anticipating associated disorders, and predicting disease risk in relatives or reoccurrence in offspring. Although clinical differences at the time of diagnosis usually allow the presumptive classification of patients as T1DM or T2DM, the classification is not always clear-cut. Likewise, it is important to know when to consider one of the more unusual forms of diabetes, as differentiating them from the more common type 1 and type 2 diabetes can present a significant diagnostic challenge.


A complete discussion of the different types of diabetes is presented in the subsequent parts of this chapter. Part B: Type 1 Diabetes and Part C: Type 2 Diabetes present the epidemiology, specific clinical presentations, and treatments of type 1 and type 2 diabetes, respectively. Part D: Other Specific Types of Diabetes Mellitus and Causes of Hyperglycemia, discusses diabetes in infants less than 6 months at diagnosis as well as the differential diagnosis of patients with an autosomal dominant or maternally transmitted patterns of diabetes inheritance as well as diabetes associated with medication or other endocrine disorders.

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