Type 1 diabetes mellitus

Overview 
Theory 
Diagnosis 
Management 
Follow up 
Resources 

Last reviewed: 21 Jun 2025

Last updated: 19 Jun 2025

Type 1 diabetes mellitus is a metabolic disorder characterized by hyperglycemia due to absolute insulin deficiency.

Patients most often present with a few days or weeks of polyuria, polydipsia, weight loss, and weakness.

Some patients may present with diabetic ketoacidosis.

Insulin is required for the treatment of type 1 diabetes.

Intensive glycemic control has been shown to decrease the incidence of microvascular and macrovascular complications.

Microvascular complications include retinopathy, nephropathy, and neuropathy.

Macrovascular complications include coronary artery, cerebrovascular, and peripheral vascular disease.

Technologic advances including insulin pumps, continuous glucose monitoring, and automated insulin delivery/hybrid closed-loop devices have led to an improvement in glycemic control and quality of life for people with type 1 diabetes.

Teplizumab may be used to delay the onset of clinical type 1 ԁiabеteѕ in individuals with preclinical disease.

Definition

Type 1 diabetes mellitus is a metabolic disorder characterized by hyperglycemia due to absolute insulin deficiency. The condition develops due to destruction of pancreatic beta cells, mostly by immune-mediated mechanisms. In some patients there may be no evidence of autoimmune destruction of pancreatic beta cells; this is called idiopathic type 1 diabetes.

History and exam

Key diagnostic factors

polyuria
polydipsia
diabetic ketoacidosis

Full details

Other diagnostic factors

young age
weight loss
blurred vision
nausea and vomiting
abdominal pain
tachypnea
clinical dehydration
lethargy
altered mental status

Full details

Risk factors

genetic predisposition
geographic region
infectious agents
dietary factors
use of immunotherapy

Full details

Diagnostic tests

1st tests to order

HbA1c
fasting plasma glucose
2-hour post-glucose load plasma glucose
random plasma glucose

Full details

Tests to consider

plasma or urine ketones
random C-peptide
autoimmune markers

Full details

Treatment algorithm

ONGOING

nonpregnant

pregnant

Contributors

Authors

Rajesh K. Garg, MD

Professor of Medicine

David Geffen School of Medicine at UCLA

Harbor-UCLA Medical Center

Torrance

Disclosures

RKG is an author of a number of references cited in this topic.

Acknowledgements

Dr Rajesh K. Garg would like to gratefully acknowledge Dr Varsha Vimalananda, previous contributor to this topic.

Disclosures

VV declares that she has no competing interests.

Peer reviewers

Zachary Bloomgarden, MD

Clinical Professor

Medicine/Endocrinology

Diabetes and Bone Disease

Mount Sinai School of Medicine

New York

Disclosures

ZB declares that he has no competing interests.

Alicia Jenkins, MB, BS, MD, FRACP, FRCP

Associate Professor

Department of Medicine

University of Melbourne

Melbourne

Australia

Professor

Harold Hamm Oklahoma Diabetes Center

University of Oklahoma Health Sciences Center

Oklahoma City

Disclosures

AJ has been a (non-salaried) co-investigator on multi-center clinical trials supported by Novo, Eli Lilly, Sanofi-Aventis, and Medtronic. She does not hold any stocks or shares in these companies. She has received a speaker's honorarium from Novo Nordisk.

References

Our in-house evidence and editorial teams collaborate with international expert contributors and peer reviewers to ensure that we provide access to the most clinically relevant information possible.

Key articles

American Diabetes Association. Standards of care in diabetes - 2025. Diabetes Care. 2025 Jan;48(1):S344-52.Full text

Norris JM, Johnson RK, Stene LC. Type 1 diabetes-early life origins and changing epidemiology. Lancet Diabetes Endocrinol. 2020 Mar;8(3):226-38.Full text Abstract

International Society for Pediatric and Adolescent Diabetes. ISPAD clinical practice consensus guidelines 2024. Dec 2024 [nternet publication].Full text

Holt RIG, DeVries JH, Hess-Fischl A, et al. The management of type 1 diabetes in adults. A consensus report by the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD). Diabetologia. 2021 Dec;64(12):2609-52.Full text Abstract

Reference articles

A full list of sources referenced in this topic is available to users with access to all of BMJ Best Practice.

Differentials
- Monogenic diabetes: maturity onset diabetes of the young
- Monogenic diabetes: neonatal diabetes mellitus
- Type 2 diabetes
More Differentials
Guidelines
- Standards of care in diabetes - 2025
- Management of individuals with diabetes at high risk for hypoglycemia
More Guidelines
Patient information
Diabetes type 1: what is it?
Diabetes type 1: what treatments work?
More Patient information
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Type 1 diabetes mellitus

Summary

Definition

History and exam

Key diagnostic factors

Other diagnostic factors

Risk factors

Diagnostic tests

1st tests to order

Tests to consider

Treatment algorithm

nonpregnant

pregnant

Contributors

Authors

Rajesh K. Garg, MD

Disclosures

Acknowledgements

Disclosures

Peer reviewers

Zachary Bloomgarden, MD

Disclosures

Alicia Jenkins, MB, BS, MD, FRACP, FRCP

Disclosures

References

Key articles

Reference articles

A full list of sources referenced in this topic is available to users with access to all of BMJ Best Practice.

Differentials

Guidelines

Patient information

Log in or subscribe to access all of BMJ Best Practice

Log in or subscribe to access all of BMJ Best Practice

Log in to access all of BMJ Best Practice