Written by Olivia Hunt
29th Sep 2022 • 5 minute read
Dr Nadja Auerbach
Reviewed by
Dr Nadja Auerbach, MBBS BSc Dip IBLM/BSLM

High blood sugar (hyperglycaemia) is when your blood sugar level is too high.  It’s typically found in people with diabetes or pre-diabetes due to difficulty in controlling their blood sugar levels. It’s essential to see your GP if you have persistently high blood sugar, as it can lead to severe health complications. Eating a balanced diet and getting enough exercise are the best ways to maintain normal blood sugar levels. Find out more about high blood sugar and what you can do to manage your levels.

What is blood sugar?

Blood sugar is the amount of glucose (a type of sugar) in your bloodstream. It’s also referred to as serum glucose level or blood glucose level.

Glucose is the primary source of energy for both your brain and body. Your body stores glucose as glycogen in your liver, skeletal muscles, and a small amount in your brain.

What are high blood sugar levels?

There are a couple of different tests to check your blood sugar levels, including:

HbA1c blood test 

An HbA1c blood test checks your blood sugar control over the last 3 months

This test can be helpful if you want to check your risk of prediabetes or diabetes. You don’t have to visit your GP for this type of test — you can do it at home using a finger-prick lancet to collect your sample, and you’ll get results within 48 hours.

You should aim to do this test before 10 am for an accurate reading, but you don’t need to fast beforehand. 

The ranges for HbA1c levels are:

  • normal range = below 42 mmol/mol
  • prediabetes = 42-47 mmol/mol
  • indicates diabetes = 48 mmol or over

Find out more about HbA1c and how to test it.

Fasting glucose test 

A fasting glucose blood test measures your blood sugar levels after an overnight fast (not eating). 

A health professional carries out this test by collecting a venous sample (through your vein). 

A reading over 7 mmol/L is high for this type of test and usually is diagnostic for diabetes.  A level between 5.5 and 6.9mmol/L is also considered high and can indicate a risk of progression to type 2 diabetes. This is sometimes called pre-diabetes or impaired glucose tolerance. 

It’s important to note that blood sugar levels differ from person to person, depending on your age and whether or not you have diabetes. Your GP can give you more advice on this if you’re concerned. 

Why is it important to keep blood sugar under control?

It’s important to control your blood sugar levels because if they become too low (hypoglycaemia) or too high (hyperglycaemia), they can have serious short and long-term side effects. Because of this, your body works to keep your blood sugar levels steady at all times — a process called homeostasis. This is achieved by your pancreas releasing two hormones, insulin and glucagon. Insulin works to reduce blood sugar levels and glucagon to increase them.

If your blood sugar level is too low, your body produces glucagon to raise your sugar levels back to normal.

If your blood sugar level is too high, your body will produce more insulin. Insulin stimulates your cells to take up sugar from your blood — which they use for energy.

What causes high blood sugar levels?

High blood sugar levels happen when your body can't use insulin to process the sugar in your blood. Or it can't produce enough insulin on its own — the underlying mechanism of diabetes. 

If you have diabetes, your  blood sugar levels can increase for various reasons, including:

  • eating a carbohydrate-rich meal
  • not exercising enough
  • having an infection or illness
  • changes in your hormone levels — like during your menstrual cycle 
  • stress

It's important to note that glucose isn’t bad for you. You need glucose to function — it’s your body's and brain's primary energy source.

What are the symptoms of high blood sugar?

It can be challenging to spot the symptoms of high blood sugar — they typically come on gradually and sometimes only when your blood sugar is exceptionally high.

The most common symptoms include:

  • feeling very thirsty
  • peeing a lot
  • feeling weak or tired
  • blurred vision
  • losing weight

How to lower blood sugar levels

The good news is that you can follow many healthy lifestyle habits to keep your blood sugar levels within a healthy range. These include:

  • taking any prescribed diabetes medication — try not to skip any days
  • avoiding overeating sugary or starchy food
  • managing stress — like deep breathing, mindfulness, and prioritising your sleep
  • exercising regularly — always check with your doctor first before increasing how much you do or trying a new activity
  • losing weight if you're overweight

What are the complications of high blood sugar?

It’s typically nothing to worry about if your blood sugar is high for a short time.

But having regular high blood sugar levels can cause serious problems, including:

  • peripheral neuropathy — permanent damage to the nerves in your hands and feet 
  • diabetic retinopathy — permanent damage to your eyes and problems with your sight 
  • diabetic ketoacidosis — a life-threatening condition where your body starts to run out of insulin

If you notice your blood sugar levels are often above 10mmol, you should seek advice from your doctor or diabetes care team. 

You should seek immediate help if:

  • you're feeling sick, being sick, or have stomach pain
  • you're breathing more quickly than usual
  • your heart is beating faster than usual
  • you feel drowsy or are struggling to stay awake
  • you feel confused or have difficulty concentrating
  • you have a high level of ketones (a chemical your liver produces to break down fats) in your blood or pee