A 52-year-old man with a 10-year history of type 2 diabetes presents with fasting blood glucose of 180 mg/dL despite metformin therapy. Which is the most common cause of persistent fasting hyperglycemia in this patient?

Explanation

## Fasting Hyperglycemia in Type 2 Diabetes **Key Point:** Fasting hyperglycemia is predominantly driven by excessive hepatic glucose production (HGP) due to loss of insulin's suppressive effect on gluconeogenesis and glycogenolysis. ### Pathophysiology of Fasting Glucose Elevation 1. **Loss of insulin-mediated suppression of HGP** — insulin normally inhibits gluconeogenesis and glycogenolysis; in type 2 diabetes, this effect is blunted 2. **Increased glucagon signaling** — relative hyperglucagonemia (due to impaired glucose sensing by alpha cells) drives HGP 3. **Increased substrate availability** — elevated free fatty acids (from adipose tissue lipolysis) and amino acids fuel gluconeogenesis 4. **Impaired hepatic glucose uptake** — reduced glycogen synthesis due to insulin resistance in the liver ### Contribution of Different Mechanisms to Fasting Hyperglycemia | Mechanism | Contribution | Timing | Reversibility | |-----------|--------------|--------|---------------| | **↑ Hepatic Glucose Production** | 80–90% | Fasting state | Partially reversible with insulin/GLP-1 RA | | ↓ Hepatic Glucose Uptake | 10–20% | Fasting state | Reversible with glycemic control | | ↓ Skeletal Muscle Uptake | Minimal in fasting | Fed state | Relevant for postprandial hyperglycemia | | ↓ Beta Cell Secretion | Secondary effect | Progressive | Not primary cause of fasting hyperglycemia | **High-Yield:** Fasting hyperglycemia reflects **hepatic pathology** (excessive production), while postprandial hyperglycemia reflects **peripheral insulin resistance** (reduced skeletal muscle uptake). **Clinical Pearl:** Metformin works primarily by suppressing hepatic glucose production via AMPK activation; failure of fasting glucose control on metformin monotherapy suggests need for agents that enhance insulin secretion (sulfonylureas, meglitinides) or GLP-1 receptor agonists. **Mnemonic:** **HGP** — Hepatic Glucose Production is the main driver of fasting hyperglycemia; Glucagon excess and insulin deficiency are secondary.