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Causes of high blood sugar in insulin therapy

If insulin's efficiency is less than the need for insulin, the blood sugar level increases above target levels.

If diabetes management is inadequate, blood sugar levels will inevitably be too high. In addition, many factors increase the need for insulin, such as stress, weight gain, reduced physical activity, and hormonal fluctuations in the menstrual cycle for women. Also various illnesses, infections, injuries, bed rest, and cortisone medication usually elevate blood sugar levels. Sometimes, the cause of temporary blood sugar elevation may remain unclear.

However, consistently or persistently elevated blood sugar levels usually have an identifiable underlying cause that should be investigated and addressed.

The absorption of insulin products and the duration of their blood sugar-reducing effect depends on the product and individual. The absorption of long-acting basal insulins may be subject to daily variations manifesting in “inexplicable” alterations in blood sugar. Swelling or hardening of the injection site may slow down the absorption process, in which case blood sugar can easily increase too much when eating.

Rapid-acting bolus insulins take longer to start decreasing blood sugar than the bolus insulin produced by a healthy pancreas. Blood sugar can easily rise too high 1–2 hours after a meal. However, it is normally corrected within 3–4 hours, or it may even drop too low. This can especially happen if the administration of mealtime insulin occurs during or after the meal.

Insulin sensitivity, in other words, the need for insulin varies individually at different times of the day. During and after adolescence, the body normally needs insulin most just before and after waking up. The main reason for this is that the effect of the insulin-antagonistic hormones, the growth hormone produced by the pituitary gland and the cortisol produced by the adrenal gland peaks in the morning. In the early hours of morning the need for insulin may increase to a degree where increasing the basal insulin dose no longer suffices and insulin needs to be dosed with a pump.

In many people, waking up results in a so-called “waking up stress”. In practice, it means that your blood sugar starts to rise when you wake up. The same quantity of carbohydrates elevates the blood sugar level more in the morning than during the day and more insulin is needed in relation to the carbohydrates. Even without eating, sugar released from the liver may increase the blood sugar level so much that a small additional dose of bolus insulin is needed.

Blood sugar rises during an acute illness and fever and the related bed rest. If you manage your diabetes with oral medications, the dosage of the tablets may need to be increased as necessary, or insulin injections may be included in the treatment under professional guidance.

If you manage your diabetes with insulin, it is important to check blood sugar levels more frequently, about every 1-2 hours, and adjust insulin doses based on your own assessment of blood sugar levels. This can be done by taking corrective insulin doses or adding a correction dose to the mealtime insulin dose. It may also be necessary to increase the dosage of long-acting insulin if you are ill for an extended period. You can find more information in the "Sick Days with Diabetes" section of the Diabeteshub.

If blood sugar levels rise above 14 mmol/l or even lower if you are feeling unwell, measure for ketones. If you do not have a ketone meter available, contact your diabetes care provider or go to the nearest emergency department during the on-call hours.

Updated 30.9.2023