Euglycaemia

Euglycaemia is plasma glucose sitting inside the normal range, conventionally a fasting value of about 3.9 to 5.5 mmol per litre, or 70 to 99 mg per decilitre, with post-meal values under about 7.8. It is a controlled variable, not a passive one: insulin restrains hepatic glucose output and drives peripheral uptake, while glucagon, catecholamines, cortisol and growth hormone push the other way. The brain, which cannot store glucose meaningfully, consumes it continuously, which is why the lower bound is defended far more aggressively than the upper.

The diagnostic thresholds sit just outside it: fasting 5.6 to 6.9 mmol per litre defines prediabetes and 7.0 or above diabetes, as does a two-hour value of 11.1 after a 75 gram oral load or an HbA1c of 6.5 percent. The hyperinsulinaemic-euglycaemic clamp, infusing insulin at a fixed rate while glucose is titrated to hold concentration constant, remains the reference method for insulin sensitivity, with the required glucose infusion rate as the readout.

The concept earns its keep in drug evaluation, because restoring euglycaemia and lowering glucose are different objectives. Agents that act only when glucose is elevated approach the normal range and stop; agents that act regardless of glucose keep going past it. That distinction is why incretin-based therapies given alone show hypoglycaemia rates close to placebo while sulfonylureas and insulin do not.

Two misreadings are common. The first treats a normal average as euglycaemia throughout the day, when a continuous monitor can show wide excursions around a perfectly normal mean. The second forgets that the label attaches to glucose alone: euglycaemic ketoacidosis, seen with SGLT2 inhibitors, presents with glucose below the usual alarm thresholds while the patient is frankly acidotic, and the normal number is precisely what delays the diagnosis.