Low glucose (hypoglycemia): what it means, causes, symptoms and what to check next

• age 18+, fasting: 70.0 – 99.0 mg/dL

Can reflect prolonged fasting, certain medications (especially diabetes drugs), or hormonal conditions.

Full reference table, citations and testing options: Glucose biomarker page.

1. Diabetes medication — insulin, sulfonylureas (glipizide, glimepiride, glyburide) and meglitinides; missed meals, extra exercise, alcohol or kidney impairment tip the balance
2. A sample left unspun for hours — red cells keep consuming glucose in the tube, dropping it 5–10 mg/dL per hour; the most common reason for a 'low' result in a well person
3. Prolonged fasting in a lean, fit person, pregnancy, and the 'reactive' dip two to four hours after a high-carbohydrate meal in people after gastric bypass or with early insulin resistance
4. Alcohol, especially on an empty stomach — it blocks the liver's glucose production for hours
5. Adrenal insufficiency (Addison's) and pituitary failure — too little cortisol or growth hormone to support glucose; also severe hypothyroidism
6. Severe liver disease, kidney failure, sepsis, malnutrition and anorexia — the liver cannot make or store enough glucose
7. Medications other than diabetes drugs — quinine, high-dose aspirin, some antibiotics (fluoroquinolones, pentamidine), beta-blockers masking symptoms, and accidental or hidden use of insulin or sulfonylureas
8. Insulinoma and other tumors, insulin autoimmune syndrome, and inherited enzyme defects — rare, with low glucose during fasting and very high insulin at the time

• Shakiness, sweating, a pounding or racing heart, hunger and anxiety — the adrenaline response, usually starting around 55–70 mg/dL
• Difficulty concentrating, confusion, blurred or double vision, slurred speech, odd behavior — the brain running short, typically below about 55
• Headache, weakness and tiredness after the episode
• Seizure or loss of consciousness when severe; symptoms during sleep show up as night sweats, nightmares and morning headaches
• None at all in some people with long-standing diabetes who have lost the warning signs (hypoglycemia unawareness)

• First, decide whether it was real: were there symptoms at the time, did they resolve with sugar, and was the sample handled promptly? Whipple's triad — low glucose, symptoms, relief on correction — is the standard
• In someone on insulin or a sulfonylurea: a review of doses, meal timing, exercise, alcohol and kidney function with their clinician — this is the cause until proven otherwise
• In someone not on those drugs: a glucose with insulin, C-peptide, proinsulin and beta-hydroxybutyrate drawn during a symptomatic low, which separates an insulin-driven cause from the rest
• Morning cortisol (and ACTH), thyroid tests, liver and kidney function, and a medication and alcohol history
• A sulfonylurea screen when insulin is high but C-peptide is also high and there is access to the drugs
• A supervised 72-hour fast and imaging in an endocrine unit when an insulinoma is suspected

Follow-up tests available at TestWell: Cortisol Test (AM) ($26.49), C-Peptide Test ($29.99).

The ADA suggests re-screening at least every 3 years when fasting glucose is normal, and at least once a year with prediabetes (100–125 mg/dL). A single fasting result at or above 126 mg/dL is confirmed with a repeat test — or an A1c — before it counts as diabetes.