Hyperglycemia

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I now have comprehensive material from Harrison's and Rosen's. Let me compile the full answer.

Hyperglycemia - Definition, Causes & Pathophysiology

Definition

Hyperglycemia refers to an abnormally elevated blood glucose concentration. It is the shared phenotype of all forms of diabetes mellitus (DM) and the central metabolic derangement from which downstream complications arise. Diagnostically, the thresholds are:

Test	Normal	Prediabetes (Impaired)	Diabetes
Fasting plasma glucose (FPG)	< 100 mg/dL (5.6 mmol/L)	100-125 mg/dL (5.6-6.9 mmol/L)	≥ 126 mg/dL (7.0 mmol/L)
2-h post-OGTT glucose	< 140 mg/dL	140-199 mg/dL	≥ 200 mg/dL (11.1 mmol/L)
HbA1c	< 5.7%	5.7-6.4%	≥ 6.5%
Random glucose + symptoms	-	-	≥ 200 mg/dL

DM is formally defined as the level of glycemia at which diabetes-specific complications (e.g., retinopathy) occur - not merely a statistical deviation from a population mean. - Harrison's Principles of Internal Medicine 22E

Normal Glucose Homeostasis (Context for Pathophysiology)

Maintaining normal plasma glucose requires precise matching of glucose use, endogenous glucose production, and dietary intake. Five key hormones regulate this balance:

Insulin (from pancreatic beta cells) - the primary glucose-lowering hormone. It suppresses hepatic glucose production, stimulates GLUT4-mediated glucose uptake in muscle and fat, promotes glycogen synthesis, lipogenesis, and protein synthesis via the PI3-kinase - IRS signaling cascade. About 50% of secreted insulin is cleared by the liver on first pass.
Glucagon (alpha cells) - released in response to hypoglycemia, stress, trauma, infection, exercise, and starvation. Rapidly increases hepatic glycogenolysis and gluconeogenesis.
Epinephrine / Norepinephrine - stimulate hepatic glucose production and limit peripheral glucose uptake via alpha- and beta-adrenergic mechanisms.
Cortisol - increases hepatic gluconeogenesis and inhibits skeletal muscle glucose uptake. Major contributor to stress hyperglycemia.
Growth hormone - antagonizes insulin action.

When any of these counterregulatory hormones are elevated, or when insulin secretion or action is impaired, hyperglycemia results. - Rosen's Emergency Medicine

Causes of Hyperglycemia

1. Type 1 Diabetes Mellitus

Autoimmune destruction of pancreatic beta cells leads to absolute insulin deficiency. The process is driven by genetic susceptibility (HLA associations), environmental triggers, and islet-directed autoimmunity (autoantibodies against islet cell antigens, GAD, IA-2, ZnT8). Beta cell mass declines progressively over years (Stage 1: autoantibodies, normoglycemia → Stage 2: dysglycemia → Stage 3: overt diabetes/DKA). Without insulin, glucose cannot enter muscle or fat cells, hepatic glucose output goes unchecked, and ketogenesis occurs.

2. Type 2 Diabetes Mellitus

A heterogeneous group of disorders involving:

Insulin resistance - peripheral tissues (muscle, fat, liver) respond poorly to insulin. The PI3-kinase / GLUT4 axis is impaired. This is the first detectable defect, heralded by postprandial hyperinsulinemia, then fasting hyperinsulinemia.
Impaired insulin secretion - progressive beta cell dysfunction fails to compensate for insulin resistance. Relative insulin deficiency ensues.
Increased hepatic glucose production - gluconeogenesis and glycogenolysis are inadequately suppressed.

In T2DM, decreased insulin sensitivity also prevents suppression of lipolysis in adipose tissue, raising circulating free fatty acids, which further worsen insulin resistance. - Harrison's and Creasy & Resnik's Maternal-Fetal Medicine

3. Other Specific Causes (Secondary Hyperglycemia)

Category	Examples
Endocrinopathies	Cushing's syndrome (excess cortisol), acromegaly (excess GH), pheochromocytoma (excess catecholamines), glucagonoma, hyperthyroidism
Pancreatic disease	Chronic pancreatitis, cystic fibrosis, hemochromatosis, pancreatectomy - islet destruction from exocrine pathology
Monogenic diabetes (MODY)	Single-gene defects in beta cell function (HNF-1α, glucokinase, HNF-4α, etc.) - autosomal dominant, early onset
Drugs/medications	Glucocorticoids, antipsychotics (atypical), tacrolimus/cyclosporine, thiazide diuretics, protease inhibitors, some chemotherapy agents
Stress hyperglycemia	Surgery, critical illness, sepsis - surge of counterregulatory hormones (cortisol, catecholamines, glucagon) overrides insulin action, even in non-diabetic patients
Insulin receptor defects	Severe insulin resistance syndromes (mutations in the insulin receptor gene)
Mitochondrial mutations	Mitochondrial DNA defects impair beta cell energy metabolism
Gestational diabetes	Placental hormones (hPL, progesterone, cortisol) produce physiological insulin resistance exceeding secretory reserve

Pathophysiologic Mechanisms at the Cellular Level

In insulin deficiency or resistance:

Reduced GLUT4 translocation - skeletal muscle and adipose tissue cannot take up glucose.
Unrestrained hepatic glucose output - both glycogenolysis and gluconeogenesis proceed without inhibition.
Lipolysis - insulin normally suppresses hormone-sensitive lipase; without it, free fatty acids flood the circulation, serving as gluconeogenic substrates and further impairing insulin signaling (lipotoxicity).
Glucotoxicity - chronic hyperglycemia itself impairs beta cell function (glucose toxicity), creating a self-perpetuating cycle.

Downstream effects of sustained hyperglycemia (leading to complications):

Increased reactive oxygen species (ROS)
Activation of the polyol pathway (glucose → sorbitol → fructose), depleting NADPH
PKC activation via de novo synthesis of diacylglycerol
Hexosamine pathway alterations
Non-enzymatic glycation - formation of advanced glycosylation end products (AGEs)

These pathways collectively underlie the microvascular and macrovascular complications of diabetes. - Goldman-Cecil Medicine / Brenner and Rector's The Kidney

Glucose Threshold and Osmotic Effects

When plasma glucose exceeds the renal tubular reabsorption threshold (~180 mg/dL for SGLT2 transporters), glucosuria occurs. This creates an osmotic diuresis - water follows glucose into the urine, resulting in:

Polyuria
Polydipsia (compensatory)
Dehydration and weight loss
In severe cases: hypotension, vascular collapse

Early manifestations of hyperglycemia include weakness, polyuria, polydipsia, altered vision, and mild dehydration. Prolonged severe hyperglycemia with metabolic acidosis manifests as Kussmaul breathing, stupor, coma, hypotension, and cardiac arrhythmias. - Medical Physiology (Boron & Boulpaep)

ELI10

Hyperglycemia - Explained Like You're 10

What is it?

Your blood is like a highway, and sugar (glucose) is like cars driving on it. Your body needs just the right number of cars - not too few, not too many. Hyperglycemia means there are WAY too many sugar-cars clogging up the highway.

Where does blood sugar come from?

Two places:

Food you eat - when you eat bread, fruit, or anything sweet, your gut breaks it into sugar and dumps it into your blood.
Your liver - your liver can make sugar and release it into your blood, even when you haven't eaten.

Why doesn't the body just fix it?

Normally it does! Your pancreas (an organ near your stomach) has special cells called beta cells. When they see too much sugar in the blood, they release a hormone called insulin.

Think of insulin as a key. Your body's cells have locks on their doors. Insulin unlocks those doors so sugar can walk inside and be used as energy. Without the key, sugar just piles up in the blood outside the locked doors.

So what goes wrong?

There are two main problems:

Problem 1 - No key (Type 1 Diabetes) Your immune system accidentally destroys the beta cells that make insulin. No beta cells = no keys = sugar stuck in the blood. Like losing every key to every door in your house.

Problem 2 - Broken locks (Type 2 Diabetes) You still have keys (insulin), but the locks on your cells are rusty and stiff - they don't open easily anymore. This is called insulin resistance. Your pancreas tries to make MORE keys to compensate, but eventually gets tired and can't keep up.

Other things that can cause it

Stress or being sick - your body releases hormones like adrenaline and cortisol (the "danger" hormones). These tell your liver to dump extra sugar into the blood for emergency energy - but if insulin can't handle it all, blood sugar spikes.
Certain medicines - steroids (like prednisone) make your cells ignore insulin.
Other hormone problems - diseases where your body makes too much cortisol or growth hormone can block insulin from working.

What happens when sugar piles up too high?

Your kidneys act like a filter. Normally they keep sugar in your blood. But when there's TOO much sugar, it spills into your urine (like a cup overflowing). Sugar in the urine drags water along with it, so you:

Pee a lot
Get really thirsty
Feel tired and weak (your cells are starving even though there's tons of sugar - it just can't get in)
Lose weight

If it gets really severe and goes untreated, it can make you very sick - even unconscious.

Quick Summary

What	Simple Version
Hyperglycemia	Too much sugar in the blood
Insulin	The key that lets sugar into cells
Type 1	Body destroys the key-makers
Type 2	Locks on cells get rusty and stiff
Stress hyperglycemia	Danger hormones flood blood with extra sugar
Symptoms	Peeing lots, thirsty, tired, blurry vision

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