The C-peptide test measures the level of C-peptide in the blood, which is a byproduct of insulin production. When the pancreas produces insulin, it is initially made as a single chain called proinsulin. Proinsulin is then split into insulin and C-peptide before being released into the bloodstream in equal amounts. Therefore, the level of C-peptide serves as an indirect marker of insulin production. This test is particularly useful in distinguishing between types of diabetes, assessing insulin production in people with diabetes, and determining the cause of hypoglycemia (low blood sugar). The relevance of the C-peptide test to testosterone replacement therapy (TRT) and men's health lies in the relationship between testosterone levels and metabolic health. Low testosterone levels have been associated with an increased risk of developing metabolic syndrome and type 2 diabetes, conditions characterized by insulin resistance and impaired glucose metabolism. Monitoring C-peptide levels in men undergoing TRT may provide insights into their metabolic status and help manage potential risks associated with metabolic disorders. Understanding the interplay between testosterone, insulin, and glucose metabolism is crucial for optimizing health outcomes in men with testosterone deficiency and related metabolic conditions.