Diabetes Mellitus, Type 1

Type 1 diabetes mellitus is a chronic disease caused by immune-mediated destruction of the insulin-producing beta cells of the pancreatic islets, leading to absolute insulin deficiency and lifelong dependence on exogenous insulin. It typically presents in childhood or early adulthood but can arise at any age, and it is biochemically defined by hyperglycaemia together with markers of islet autoimmunity.

Definition

Type 1 diabetes mellitus is a form of diabetes resulting from autoimmune destruction of pancreatic beta cells, producing absolute insulin deficiency and a consequent requirement for exogenous insulin to sustain life.

Scope

The entry describes type 1 diabetes as a pathological and laboratory-defined entity: its autoimmune mechanism, the natural history from preclinical autoimmunity to symptomatic disease, the autoantibodies and metabolic measurements used to characterise it, and its distinction from type 2 diabetes. It does not provide insulin regimens or individualised management advice.

Key concepts

• Beta-cell autoimmunity
• Absolute insulin deficiency
• Islet autoantibodies (GAD65, IA-2, ZnT8, insulin)
• Genetic susceptibility (HLA class II)
• Preclinical staging of disease
• Diabetic ketoacidosis as a presenting state
• C-peptide as a marker of endogenous insulin

Mechanisms

Type 1 diabetes develops when an autoimmune process, triggered in genetically susceptible individuals, progressively destroys the insulin-secreting beta cells of the pancreatic islets. The disease is associated with particular HLA class II haplotypes and is heralded by circulating autoantibodies against islet antigens such as glutamic acid decarboxylase (GAD65), insulinoma-associated antigen 2 (IA-2), zinc transporter 8 (ZnT8), and insulin itself. As beta-cell mass falls, insulin secretion becomes insufficient to control blood glucose, and once a critical threshold is crossed, symptomatic hyperglycaemia and, if untreated, diabetic ketoacidosis ensue. Because insulin deficiency is absolute rather than relative, endogenous insulin (reflected by C-peptide) is low or absent, distinguishing the disease from type 2 diabetes.

Clinical relevance

Type 1 diabetes is a principal cause of diabetes in young people and a model autoimmune endocrine disease, and its laboratory characterisation through glucose, HbA1c, autoantibodies, and C-peptide is foundational in clinical chemistry. This entry explains how the disease is defined and classified for reference; it is not a source of treatment recommendations for any individual.

Epidemiology

Type 1 diabetes accounts for a minority of all diabetes cases but is the predominant form in children and adolescents, with incidence varying widely between populations and showing geographic and secular increases. It can present across the lifespan, and a substantial proportion of cases are now recognised to be diagnosed in adulthood.

Evidence & guidelines

The autoimmune pathophysiology and natural history are summarised in major reviews, while consensus management frameworks have been issued jointly by the American Diabetes Association and the European Association for the Study of Diabetes; these are cited here for orientation rather than as prescriptive instructions.

History

Type 1 diabetes was uniformly fatal until the isolation of insulin in 1921-1922 transformed it into a treatable chronic disease. Over subsequent decades it was recognised as distinct from type 2 diabetes, and the discovery of islet-cell autoantibodies and HLA associations established its autoimmune basis, leading to the modern concept of a staged disease that begins long before clinical onset.

Seminal works

• katsarou-2017
• atkinson-2014
• dimeglio-2018

Frequently asked questions

How does type 1 diabetes differ from type 2 diabetes?

Type 1 diabetes is caused by autoimmune destruction of beta cells producing absolute insulin deficiency, whereas type 2 diabetes is driven mainly by insulin resistance with relative insulin deficiency; the two differ in mechanism, typical age of onset, and laboratory markers such as autoantibodies and C-peptide.

What laboratory findings characterise type 1 diabetes?

It is characterised by hyperglycaemia with low or absent C-peptide and, in most cases, the presence of one or more islet autoantibodies such as GAD65, IA-2, ZnT8, or insulin antibodies.