When you eat carbohydrates, your blood glucose rises, and your pancreas releases insulin to shuttle that glucose into cells. This is a normal, healthy process. Insulin resistance occurs when cells stop responding appropriately to insulin's signal — the pancreas produces more insulin to compensate, blood glucose stays elevated, and circulating insulin levels remain chronically high. This state — hyperinsulinemia — is one of the most consequential hormonal disruptions in modern medicine, and it reaches far beyond blood sugar.
How Insulin Resistance Disrupts Your Hormones
Testosterone in men: Insulin regulates the liver's production of SHBG (sex hormone-binding globulin). High insulin suppresses SHBG, which reduces total testosterone through feedback mechanisms, and high insulin also directly impairs testosterone synthesis in the Leydig cells of the testes. Men with insulin resistance consistently have lower total and free testosterone than insulin-sensitive men of the same age — a relationship that persists even after adjusting for body weight and other confounders.
Estrogen and PCOS in women: In women, high insulin stimulates the ovaries to produce excess androgens (testosterone and its precursors). This androgen excess suppresses ovulation, disrupts cycle regularity, and drives the clinical features of PCOS — acne, hirsutism, irregular cycles, and fertility challenges. Simultaneously, excess androgens are peripherally converted to estrogen in fat tissue, driving estrogen excess even as ovarian function is impaired. The result is a hormonal environment characterized by both androgen excess and disrupted estrogen signaling.
Thyroid function: Insulin resistance impairs T4-to-T3 conversion (the same conversion suppressed by chronic stress), reduces thyroid hormone receptor sensitivity, and directly inhibits thyroid peroxidase activity. Chronically insulin-resistant individuals frequently have suboptimal thyroid function even when TSH appears normal.
Cortisol amplification: Insulin resistance and cortisol exist in a self-reinforcing cycle. Elevated cortisol drives insulin resistance through multiple mechanisms — glucose release from the liver, fat redistribution to visceral depots, and reduced insulin receptor sensitivity. Insulin resistance in turn elevates cortisol through adipose tissue inflammation and HPA axis stimulation. Breaking this cycle requires addressing both simultaneously.
Measuring Insulin Resistance
Fasting glucose is an insensitive marker of insulin resistance — it is often normal until insulin resistance is advanced. More useful markers include: fasting insulin (optimal: below 8 μIU/mL), HOMA-IR (calculated from fasting glucose and insulin; optimal below 1.5), triglyceride-to-HDL ratio (a surrogate of insulin sensitivity; optimal below 1.5), and hemoglobin A1c (optimal below 5.4% for metabolic health, not just "not diabetic"). Continuous glucose monitoring provides the most detailed picture of glucose metabolism in response to diet and lifestyle.
Addressing Insulin Resistance
The most evidence-based interventions for insulin resistance are: resistance training (increases glucose uptake into muscle independent of insulin), zone 2 aerobic exercise (improves mitochondrial function and insulin sensitivity), dietary carbohydrate reduction (particularly of refined carbohydrates and added sugars), sleep optimization (poor sleep acutely impairs insulin sensitivity), and reduction of chronic stress (via HPA axis mechanisms above).
GLP-1 receptor agonists are among the most powerful pharmacological tools for insulin sensitization — they work through mechanisms beyond weight loss alone, improving insulin secretion kinetics, reducing hepatic glucose output, and improving peripheral insulin sensitivity. For patients with significant insulin resistance, they represent a meaningful adjunct to lifestyle interventions.