PassFinals
Endocrinology & Metabolic

Type 1 Diabetes Mellitus

An autoimmune disease that destroys pancreatic beta cells until insulin secretion fails almost completely, so glucose rises unchecked and unopposed lipolysis drives ketone production: the mechanism behind both the presentation and the risk of diabetic ketoacidosis.

In a nutshell

Autoimmune beta-cell destruction causes absolute insulin deficiency: osmotic symptoms and weight loss, with unopposed lipolysis creating a lifelong DKA risk. Management is immediate insulin replacement plus structured self-management, ketone sick-day rules, hypoglycaemia prevention, glucose technology, pregnancy planning and annual complication review.

Classic presentation

A person with rapid-onset thirst, polyuria, weight loss and fatigue, sometimes presenting with DKA; do not use age, BMI or an atypical phenotype alone to exclude type 1 diabetes.

Key points

  • Absolute insulin deficiency, not insulin resistance, is the defining lesion; insulin is required from diagnosis and must continue during illness even when oral intake falls.
  • Do not use age or BMI alone to diagnose or exclude adult type 1 diabetes; autoantibodies support classification but negative tests do not exclude it.
  • A basal-bolus regimen with rapid-acting mealtime insulin is the default replacement strategy; current UK supply advice means new patients should not be started on detemir.
  • Support an individualised HbA1c target of 48 mmol/mol (6.5%) or lower only if this does not cause problematic hypoglycaemia.
  • Offer all adults a choice of rtCGM or isCGM; hybrid closed-loop systems have a NICE pathway for adults with persistent poor control or disabling hypoglycaemia.
  • Illness, hyperglycaemia or pump failure requires ketone testing and a sick-day plan; vomiting, Kussmaul breathing, drowsiness or rising ketones is an emergency.
  • Treat a conscious hypo with rapid carbohydrate; reduced consciousness requires glucagon or IV glucose, observation and review of the insulin plan.
  • Pregnancy requires preconception optimisation, ketone access, frequent glucose monitoring and specialist joint diabetes-antenatal care.

First-line investigation

Confirm hyperglycaemia on clinical grounds and check blood or urine ketones immediately if the person is unwell, hyperglycaemic or has DKA symptoms; use autoantibodies and specialist classification tests when the subtype is uncertain.

Management

Start insulin and recognise DKA

  • Start insulin at diagnosis. Use basal-bolus replacement with a rapid-acting analogue before meals and a long-acting basal insulin selected with the diabetes team. Do not initiate new detemir while Levemir is being discontinued in the UK; follow available-product and local specialist advice.1,6,3
  • Vomiting, abdominal pain, Kussmaul breathing, dehydration, reduced consciousness or rising ketones suggests DKA: check ketones and acid-base status urgently, admit for the adult DKA pathway and do not attempt to manage severe deterioration by adjusting outpatient insulin alone.1,4,2

Set targets and teach core self-management

  • Support an individualised HbA1c target of 48 mmol/mol (6.5%) or lower without problematic hypoglycaemia; measure HbA1c every 3 to 6 months. Aim for fasting glucose 5 to 7 mmol/L on waking and pre-meal glucose 4 to 7 mmol/L at other times, adjusted to the individual context.1
  • Offer structured education such as DAFNE, usually within 6 to 12 months of diagnosis, covering carbohydrate counting, insulin adjustment, exercise, alcohol, glucose data, illness and hypoglycaemia prevention.1,2

Use glucose technology safely

  • Offer all adults a choice of rtCGM or isCGM based on preferences, needs and device functionality. People using capillary monitoring need at least 4 checks daily and more during illness, sport, driving, pregnancy, hypoglycaemia or when targets are not met.1
  • For adults with HbA1c 58 mmol/mol (7.5%) or higher or disabling hypoglycaemia despite best possible management with at least one of CSII, rtCGM or isCGM, NICE TA943 recommends hybrid closed-loop systems with trained multidisciplinary support and structured education.8

Prevent insulin-related harm

  • Provide written sick-day rules: never omit basal insulin, check ketones during illness or hyperglycaemia, maintain fluids and carbohydrate where possible, adjust insulin using the agreed plan and seek urgent help for vomiting, worsening ketones, respiratory distress or reduced consciousness.1,2,4
  • Prescribe and administer the correct insulin product and strength by brand or device; never withdraw insulin from a pen or cartridge into a syringe. Pump users need a backup injection and ketone plan for infusion failure.7,3,2

Treat hypoglycaemia and manage impaired awareness

  • If conscious and able to swallow, give 15 to 20 g rapid-acting carbohydrate, recheck after 10 to 15 minutes and repeat if still low, then provide longer-acting carbohydrate or a meal. If consciousness is reduced, use IM glucagon by a trained person or IV glucose by a healthcare professional and observe for relapse.1,9,10
  • Assess hypoglycaemia awareness at annual review. Recurrent severe hypoglycaemia or impaired awareness warrants structured education, regimen review, CGM or pump discussion and specialist referral; do not simply relax the person's glucose targets without addressing the cause.1,8

Plan driving, exercise and pregnancy

  • Insulin-treated drivers must follow DVLA requirements, monitor glucose at times relevant to driving and stop driving if hypoglycaemia or impaired awareness makes it unsafe. CGM alarms supplement but do not replace active monitoring and the ability to recognise and respond to low glucose.11,1
  • Before pregnancy, aim for HbA1c below 48 mmol/mol if achievable without problematic hypoglycaemia, offer ketone testing and folic acid 5 mg daily until 12 weeks. During pregnancy, offer rtCGM, frequent glucose monitoring, glucagon access and urgent assessment for illness or hyperglycaemia.12,8

Screen for complications and associated autoimmunity

  • At least annually review HbA1c, hypoglycaemia awareness, blood pressure and cardiovascular risk, urine albumin-to-creatinine ratio and eGFR, retinal screening, injection sites and foot risk. Consider thyroid and coeliac disease and refer abnormal findings to the relevant pathway.1,5

Exam traps

  • Never withhold insulin because a patient is not eating: omission, not food, is what precipitates ketoacidosis.
  • Weight loss with osmotic symptoms in a young, lean patient should suggest type 1, not type 2, diabetes.
  • A single normal glucose does not exclude type 1 diabetes early in the autoimmune process; recheck if suspicion remains.
  • Do not use age or BMI alone to exclude adult type 1 diabetes; autoantibodies and the clinical course guide classification.
  • A pump or hybrid closed-loop system does not remove DKA risk: infusion failure can cause rapid insulin deficiency, so ketone testing and backup insulin matter.
  • CGM alarms do not replace the ability to recognise and safely treat hypoglycaemia when driving.
  • Levemir is being discontinued in the UK; do not copy the old detemir wording into a new prescription without checking current availability and the specialist switch plan.

Illustrations

Autoimmune beta-cell destructionDiagram of T-cell mediated infiltration of a pancreatic islet, showing progressive loss of insulin-secreting beta cells with disease progression.PassFinals · original
Insulin regimen profileGraph illustrating a basal-bolus insulin regimen mimicking physiological background secretion and meal-time surges.PassFinals · original
Diabetic ketoacidosis warning signsIllustration of Kussmaul breathing, dehydration and reduced consciousness as ketoacidosis develops in insulin deficiency.PassFinals · original

Key sources

  1. NICE, Type 1 diabetes in adults: diagnosis and management (NG17)Published 26 Aug 2015 | Updated 17 Aug 2022
  2. NICE CKS, Diabetes type 1
  3. BNF, Insulin treatment summaries and insulin monographs
  4. NICE NG17, ketone monitoring and diabetic ketoacidosis recommendations (NG17 section 1.10)
  5. NICE, Diabetic foot problems: prevention and management (NG19)Published 26 Aug 2015
  6. NHS, Long-acting insulin
  7. MHRA, Patient Safety Alert: Risk of severe harm and death if insulin is withdrawn from pen devices (CAS Alert 102554)
  8. NICE, Hybrid closed loop systems for managing blood glucose levels in type 1 diabetes (TA943)Published 19 Dec 2023
  9. JBDS-IP, Hospital Management of Hypoglycaemia in Adults with Diabetes (JBDS 01)Published 1 Jan 2023
  10. BNF, Glucagon
  11. DVLA, Diabetes mellitus: assessing fitness to driveUpdated 7 Nov 2025
  12. NICE, Diabetes in pregnancy: management from preconception to the postnatal period (NG3)Published 25 Feb 2015 | Updated 16 Dec 2020

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.