Fasting insulin is one of the earliest markers of metabolic dysfunction, often elevated years before fasting glucose becomes abnormal. High fasting insulin indicates insulin resistance, which is strongly associated with low testosterone, abdominal obesity, and cardiovascular disease. Optimal fasting insulin is below 5 uIU/mL; levels above 10 suggest meaningful insulin resistance.
TRT can improve insulin sensitivity, but addressing diet, exercise, and body composition is the primary intervention.