Type 1 diabetes (T1DM, formerly insulin dependent or juvenile onset diabetes) results from autoimmune destruction of insulin-producing islet cells of the pancreas. The resulting loss of insulin secretion leads to increased blood glucose levels and increases the risk for severe high (hyperglycemia) or low (hypoglycemia) blood glucose as well as long-term complications of diabetes.

People with type 1 diabetes require insulin to control their blood sugar, given either by daily injections or insulin pump. Some people with type 1 diabetes undergo pancreas or islet transplantation, to replace insulin producing cells. However, the risks associated with these procedures must be weighed carefully for each individual.

Type 2 diabetes (T2DM, formerly non-insulin dependent, or adult onset diabetes), is the most common form of diabetes (about 90%). It is due to both reduced insulin secretion and increased resistance to insulin action, resulting in high blood sugar. It is often associated with other metabolic problems, including weight gain, high cholesterol, high blood pressure and increased risk for cardiovascular disease, such as heart attack and stroke.

T2DM often has no symptoms and may go undetected for years. Risk factors include a family history of T2DM, obesity, sedentary lifestyle, diabetes of pregnancy, and other risk factors. Anyone with one or more risk factors for T2DM should have their blood glucose checked periodically to avoid potential complications of undetected diabetes.

Treatment of type 2 diabetes includes lifestyle therapy (modified diet, regular exercise and weight loss) and may also include oral medication and/or insulin in some people. In addition to controlling glucose, it is important to treat other associated conditions, including high blood pressure and cholesterol to minimize the chance of complications.

Diabetes of pregnancy (or gestational diabetes) is an elevation of blood sugar that occurs during pregnancy. This is most often related to increased insulin resistance associated with pregnancy and an inability of the pancreas to produce enough insulin to overcome this resistance.

Testing for diabetes of pregnancy is recommended between 24-28 weeks of gestation. It can often be treated with dietary modification and physical activity, but some people also require insulin. Blood sugar control is especially important during pregnancy to avoid potential complications to the baby, as well as the mother.

Most diabetes of pregnancy resolves after delivery. However, women with diabetes of pregnancy are at risk for the development of type 2 diabetes in the future, so all women who develop diabetes of pregnancy should have their blood sugar checked after delivery and every year afterward to avoid undetected diabetes.

There are many less common causes of diabetes which do not easily fit into the above categories and account for only 5% of the total. These may include certain genetic syndromes (monogenic diabetes), pancreatic surgery, chronic pancreatitis, certain medications (e.g. steroids), and many others. If there is any question about the type of diabetes that someone has, they should see an endocrinologist, who can help clarify their diagnosis and determine the best course of treatment.