What Is Hypoglycemia?

Hypoglycemia — commonly called "a low" — occurs when your blood sugar drops below 70 mg/dL (3.9 mmol/L). At this level, there isn't enough glucose circulating in your bloodstream for your body to function at its best. Your brain, in particular, is extremely sensitive to even small drops in glucose because it has no backup fuel source.

If you have type 1 diabetes, lows are not a question of "if" — they're a question of "when." Understanding what hypoglycemia is, how it's classified, and why it happens gives you the foundation to handle every low with confidence instead of fear.

The Three ADA Classification Levels

The American Diabetes Association (ADA) and the International Hypoglycemia Study Group established a standardized classification so that patients, caregivers, and healthcare teams all speak the same language. Here are the three levels:

Quick-Reference Card: The 3 Levels at a Glance

Level 1 — ALERT (< 70 mg/dL / 3.9 mmol/L) You feel it. You can treat it. Eat 15 g fast-acting carbs now.

Level 2 — URGENT (< 54 mg/dL / 3.0 mmol/L) Thinking gets hard. Eat 15-20 g fast-acting carbs immediately. If no improvement in 15 min, repeat.

Level 3 — EMERGENCY (can't self-treat) Someone else must help. Glucagon if available. Call emergency services (911 / 112 / 15-SAMU).

Why Hypoglycemia Happens

There's no single cause of hypoglycemia. It almost always comes from a mismatch between insulin, glucose intake, and your body's current sensitivity. Here are the four main categories:

Too Much Insulin

• Overestimated carbs in a meal (you bolused for 60 g but ate 40 g)
• Stacking insulin doses — correcting a high too soon, before the previous bolus has finished working
• Basal rate too high (especially overnight or between meals)
• Correction bolus too aggressive for your current sensitivity

Not Enough Carbs

• Missed meal or snack after bolusing
• Delayed meal — the insulin kicks in before the food does
• Eating less than you bolused for (left food on the plate, felt full)
• Vomiting after bolusing (the insulin was delivered, but the carbs weren't absorbed)

Increased Insulin Sensitivity

• Physical activity — even hours after exercise, your body uses insulin more efficiently
• Alcohol consumption — alcohol blocks the liver from releasing glucose
• Weight loss or improved fitness level
• Hot weather or hot baths (heat increases absorption)

Absorption Issues

• Certain injection or infusion sites absorb insulin faster (abdomen > arm > thigh)
• Exercise increases blood flow to muscles, speeding up absorption
• Heat from a hot shower, sauna, or direct sun on the site

Scenario: Mia's First Low

Mia is 19 years old. She was diagnosed with T1D six months ago, and she's still learning the ropes. One afternoon, she's studying at the library. She bolused for lunch — a sandwich she estimated at 45 g of carbs — but she only ate half because she got a phone call and forgot to finish.

About 90 minutes later, she notices her hands are trembling. Her heart is beating faster than usual. She checks her CGM: 63 mg/dL with a downward arrow. That's Level 1.

She thinks, "I feel weird, but it's not that low. I'll just finish this chapter and then eat something." But five minutes later, the words on the page are blurring. She's having trouble following the sentence she just read. Her CGM buzzes again: 52 mg/dL. That's Level 2.

Mia pulls glucose tabs out of her bag (she keeps them in every bag — a habit her endo recommended). She eats four tabs (16 g of glucose), waits 15 minutes, and rechecks: 71 mg/dL with an upward arrow. She follows up with a small handful of crackers for sustained carbs.

What Mia learned: Never delay treatment. The jump from "I feel a little off" to "I can't think straight" happened in under five minutes. The reason? She bolused for 45 g of carbs but only ate about 20 g. The mismatch between insulin delivered and carbs absorbed created the low.