Diabetes mellitus type 1 (Type 1 diabetes, T1DM, IDDM, or, formerly, juvenile diabetes) is a form of diabetes mellitus that results from autoimmune destruction of insulin-producing beta cells of the pancreas. The subsequent lack of insulin leads to increased blood and urine glucose. The classical symptoms are polyuria (frequent urination), polydipsia (increased thirst), polyphagia (increased hunger), and weight loss.

Incidence varies from 8-17/100,000 in Northern Europe and the U.S., with a high of about 35/100,000 in Scandinavia, to a low of 1/100,000 in Japan and China.

Eventually, type 1 diabetes is fatal unless treated with insulin. Injection is the most common method of administering insulin; other methods are insulin pumps and inhaled insulin. Pancreatic transplants have been used. Pancreatic islet cell transplantation is experimental, though growing.

Most people who develop type 1 are otherwise healthy. Although the cause of type 1 diabetes is still not fully understood, it is believed to be of immunological origin.

Type 1 can be distinguished from type 2 diabetes via a C-peptide assay, which measures endogenous insulin production.

Type 1 treatment must be continued indefinitely in all cases. Treatment is not intended to significantly impair normal activities, and can be done adequately if sufficient patient training, awareness, appropriate care, discipline in testing and dosing of insulin is taken. However, treatment remains quite burdensome for many people. Complications may be associated with both low blood sugar and high blood sugar, both largely due to the non-physiological manner in which insulin is replaced. Low blood sugar may lead to seizures or episodes of unconsciousness and requires emergency treatment. High blood sugar may lead to increased fatigue and can also result in long term damage to organs.

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