Can you have high blood sugar without diabetes?

The common causes

Medications, and steroids above all. Prednisone, dexamethasone, and injected steroids for joints or the spine raise glucose substantially, and a single injection can affect readings for days to weeks. Also on the list: thiazide diuretics, some atypical antipsychotics (olanzapine, clozapine), immunosuppressants such as tacrolimus, high-dose niacin, and decongestants containing pseudoephedrine.

Acute illness and infection. Inflammatory signalling induces temporary insulin resistance. A urinary tract infection or pneumonia can push someone well into the 200s.

Surgery, trauma and critical illness. So-called stress hyperglycemia is common in hospital and usually settles.

Pancreatic problems. Chronic pancreatitis, pancreatic surgery, pancreatic cancer, or damage from hemochromatosis all reduce insulin-producing capacity.

Hormonal conditions. Cushing's syndrome (cortisol excess), acromegaly (growth hormone excess), hyperthyroidism.

Pregnancy. Gestational diabetes is common and specifically screened for.

The most likely explanation, though

Before reaching for the rare causes: the most common reason for high blood sugar in someone "without diabetes" is undiagnosed diabetes or prediabetes. Roughly 1 in 5 people with diabetes in the US do not know they have it, and most people with prediabetes are unaware.

Feeling well is entirely compatible with a fasting glucose of 115 mg/dL. So a high reading in someone with no diagnosis usually calls for a proper fasting glucose and A1c before looking for anything exotic.

The follow-up that gets missed

This is worth emphasising because it is skipped routinely.

Stress hyperglycemia during a hospital stay is typically noted, managed, and then forgotten on discharge. It should not be. People who develop it are at markedly higher risk of developing diabetes later — the illness revealed a limited reserve rather than creating a new problem.

If your glucose ran high during an illness, after surgery, or on a course of steroids, ask for a fasting glucose or A1c a few weeks after you have recovered. That single test is the whole intervention, and it is frequently never arranged.

When it is urgent

The danger thresholds do not change because you have no diagnosis. Above 300 mg/dL (16.7 mmol/L) is urgent, and above 240 mg/dL with ketones, vomiting, deep rapid breathing or confusion needs emergency care.