Diabetes mellitus is a chronic metabolic disorder characterized by hyperglycemia. The four categories of diabetes are type 1 diabetes, type 2 diabetes, other specific types of diabetes, and gestational diabetes.

Type 1 diabetes is characterized by autoimmune-mediated destruction of pancreatic β cells, with environmental factors potentially triggering this process in genetically susceptible individuals. Despite many not having a family history, certain genes increase susceptibility, suggesting a complex interplay between genetics and environment. The autoimmune destruction, often associated with a virally induced or inflammatory response, progresses over months to years, leading to significant loss of β cell mass and function. Hyperglycemia manifests once a critical threshold of β cell destruction is surpassed, signifying the clinical onset of the disease.

Type 2 diabetes differs markedly from type 1, encompassing a heterogeneous syndrome with roots in insulin resistance and impaired insulin secretion. Typically associated with obesity, and having a gradual onset, there is a strong genetic predisposition. Insulin resistance prevents efficient glucose uptake, and skeletal muscle, adipose tissue, and the liver are pivotal to its development. Elevated fasting insulin levels, increased circulatory proinsulin proportions, and dysregulated glucagon secretion complicate the condition. Intrinsic and extrinsic factors such as aging, lifestyle, and intra-abdominal fat contribute to insulin sensitivity variations.

Other forms of diabetes include monogenic forms of diabetes, such as MODY and neonatal diabetes, which arise from mutations affecting key genes in glucose regulation, presenting a range of clinical features. These can often mimic type 1 or type 2 diabetes but have distinct genetic underpinnings, necessitating tailored treatments. Additionally, diseases like pancreatitis or cystic fibrosis and certain endocrinopathies can induce secondary forms of diabetes by disrupting normal glucose metabolism.

Gestational diabetes is glucose intolerance which occurs during pregnancy. Physiologically, it's driven by placental hormones causing insulin resistance to ensure a steady glucose supply to the growing fetus. In most cases, the mother's pancreas compensates by producing more insulin. However, when insulin production is insufficient, gestational diabetes ensues. Though it typically resolves after childbirth, it increases the risk for type 2 diabetes in both mother and child in the future.

Each form of diabetes exhibits unique pathophysiological characteristics, necessitating individualized approaches to monitoring and treatment to manage the abnormal glucose homeostasis central to the disease.