My three year old son, who has type 1 diabetes, often has blood sugars that are under 4.0 mmol/L (72 mg/dl), sometimes even in the 2.5 mmol/L (45 mg/dl) range. He has very few symptoms of being low. Is any harm being done by having blood sugars this low?

You should relax on the possible consequences of the values you report since they don’t seem so low. I’d advise you to read carefully our previous answers on the topic of hypoglycemia, especially the neurological consequences and also hypoglycemia unawareness where it is explained how persistent low blood sugars, even not in the frank hypo ranges, can impair symptoms of having low blood sugar, and that in small children it is safer to keep blood sugars levels at a more ‘relaxed’ values than you report in your son.

MS
Additional comments from Dr. John Schulga:

Blood sugars of less than 4 mmol/L (72 mg/dl) and, certainly, less than 2.5 mmol/L (45mg/dl) are suggestive of needing to cut back on the insulin dose. If your son is recently diagnosed, he may be going through the honeymoon period and so should have his insulin reduced.

JS