Foundations
Hyperglycaemia: Why Glucose Goes High
Glucose rising above target is one of the most common experiences in type 1 diabetes. Understanding why it happens — and what is going on physiologically — is the starting point for knowing what to look for on your CGM and when to act.
What hyperglycaemia means
Hyperglycaemia is the term for blood glucose above the normal range. In type 1 diabetes, it is generally defined as glucose above 10 mmol/L (180 mg/dL). This is the threshold above which long-term complication risk begins to increase, and the level that most CGM time-in-range targets use as the upper boundary.
Brief excursions above 10 mmol/L are a normal part of life with type 1 diabetes. The concern is not any single spike, but rather the time spent persistently high — and the situations where highs signal something more urgent, such as a delivery failure or rising ketones.
Common causes of high glucose in type 1 diabetes
Glucose can go high for many reasons. Some are obvious; others are subtle. Recognising the common patterns is one of the most useful things CGM data can reveal.
- Missed or late bolus — the single most common cause of post-meal highs. Even a few minutes’ delay changes the insulin-to-carb timing significantly.
- Underestimated carbohydrate — particularly with meals that are difficult to count, or foods with hidden carbohydrate content.
- Infusion site or cannula failure — for pump users, a kinked or dislodged cannula can silently stop insulin delivery. This is one of the most important causes to recognise quickly.
- Illness and infection — counter-regulatory hormones rise during illness, increasing insulin resistance and hepatic glucose output. Many people find they need substantially more insulin when unwell.
- Stress hormones — cortisol and adrenaline raise blood glucose independently of food. This includes physical stress, emotional stress, and poor sleep.
- Dawn phenomenon — a rise in glucose in the early morning hours driven by growth hormone and cortisol, which increase hepatic glucose output before waking.
- Rebound from hypo treatment — overtreating a low can produce a sharp rise, particularly when glucose tabs, juice, and the liver’s own counter-regulatory response all arrive at once.
- High-fat and high-protein meals — fat slows gastric emptying and protein stimulates glucagon. The result is a delayed glucose rise that arrives hours after the meal, often after the bolus insulin has worn off.
- Insufficient basal insulin — if background insulin is not covering hepatic glucose output between meals, glucose drifts upward even without food.
What is happening physiologically
In type 1 diabetes, the beta cells that produce insulin have been destroyed. Without sufficient insulin action, two things happen simultaneously:
- Glucose cannot enter cells efficiently — insulin is the signal that allows muscle and fat cells to take up glucose from the blood. Without it, glucose accumulates.
- The liver continues to produce glucose — in the absence of insulin, hepatic glucose production is not suppressed. The liver keeps releasing glucose into the bloodstream even when blood glucose is already high.
There is an important nuance here. In someone without diabetes, insulin is secreted directly into the portal vein, which delivers it to the liver first at high concentration. This efficiently suppresses hepatic glucose output. Injected or pumped insulin, by contrast, enters the peripheral circulation and reaches the liver at a much lower concentration. This portal vein mismatch means that exogenous insulin is inherently less effective at switching off liver glucose production than endogenous insulin.
This is one of the fundamental reasons why glucose management in type 1 diabetes is so challenging — the tool available (subcutaneous insulin) does not perfectly replicate what the body originally did.
Persistent highs and the action framework
A brief spike after a meal is one pattern. A glucose level that stays above 14 mmol/L (250 mg/dL) for 90 minutes or more is a different situation entirely — and it shifts the likely cause.
When glucose is persistently high and not responding to correction insulin, the most common explanation is a delivery or site problem. For pump users, this means the insulin may not be reaching the subcutaneous tissue at all.
The priority in this situation tends to follow a consistent pattern:
- Check the infusion site for signs of failure — redness, swelling, a kinked cannula, or air in the tubing
- Change the site if there is any doubt
- Correct with a reliable delivery route — typically an insulin pen injection
The Hypo and Hyperglycaemia Explorer (Tab 3) walks through this action framework in detail, with system-specific guidance for different AID setups.
Worth noting: persistent highs that do not respond to correction are one of the earliest warning signs of developing ketones. Checking ketones at this point is a worthwhile step.
Ketones — when highs become dangerous
When there is not enough insulin for cells to use glucose, the body switches to breaking down fat for energy. This process produces ketones — acidic byproducts that accumulate in the blood.
In small amounts, ketones are normal (they are produced during fasting and exercise). In type 1 diabetes, the danger arises when ketone production accelerates because of true insulin deficiency — and the body cannot clear them fast enough.
The four ketone levels
| Blood ketone level | What it means |
|---|---|
| Below 0.6 mmol/L | Normal range. No action needed. |
| 0.6 – 1.4 mmol/L | Mildly elevated. Indicates insulin may be insufficient. Worth monitoring and addressing the cause. |
| 1.5 – 2.9 mmol/L | Significantly elevated. Risk of DKA is increasing. This typically requires prompt additional insulin and close monitoring. |
| 3.0 mmol/L or above | Dangerously high. DKA may be developing or present. This is a medical emergency — seek urgent medical attention. |
Diabetic ketoacidosis (DKA) occurs when ketone levels become high enough to make the blood acidic. It is a medical emergency that requires hospital treatment. DKA can develop within hours, particularly during illness or after a pump failure that stops insulin delivery completely.
For a deeper exploration of ketone physiology and the correction framework, see the ketones education page and the Hypo and Hyperglycaemia Explorer (Tab 3), which can calculate ketone correction doses for your specific setup.
Sick day rules — why illness changes everything
Illness is one of the most common triggers for persistent hyperglycaemia and ketone development. The mechanism is straightforward: infection and inflammation cause the body to release counter-regulatory hormones — cortisol, glucagon, adrenaline, and growth hormone — all of which increase insulin resistance and stimulate hepatic glucose output.
Many people find they need significantly more insulin during illness, not less. This can feel counterintuitive, especially when appetite is reduced and food intake is low. But the increased insulin resistance means that even without eating, glucose and ketone levels can rise rapidly.
Critical principle: never stop taking insulin during illness. Even when not eating, basal insulin is needed to suppress hepatic glucose output and prevent ketone production. Stopping insulin during illness is one of the most common pathways to DKA.
The key principles during illness tend to be consistent:
- Continue all insulin — basal and correction
- Monitor blood ketones regularly (every 2–4 hours when glucose is persistently above 14 mmol/L)
- Stay hydrated — dehydration accelerates ketone accumulation
- Many people find they need to increase basal rates or add additional correction doses
The Hypo and Hyperglycaemia Explorer (Tab 3) includes sick day protocols and can calculate ketone correction doses tailored to your AID system and insulin sensitivity. This is worth exploring with your care team before you need it.
What this means in practice
Hyperglycaemia in type 1 diabetes is not a failure — it is a consequence of the tools available not perfectly replacing what the body originally did. The portal vein mismatch, the variability of subcutaneous insulin absorption, and the complexity of the hormonal environment all contribute.
Understanding the common causes and the physiological mechanisms makes it easier to interpret CGM data, recognise patterns, and know when a high glucose reading is a routine fluctuation versus something that needs prompt attention — particularly when persistent highs and rising ketones signal a delivery problem or illness.
This content is for educational exploration only. It describes average responses and general principles. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team.
