Metabolic Effects of Testosterone Replacement Therapy in Men with Functional Secondary Hypogonadism, Obesity and Type 2 Diabetes or Metabolic Syndrome: A Systematic Review

Background/Objectives: Functional secondary hypogonadism is frequent in men with obesity, metabolic syndrome, and type 2 diabetes mellitus (T2DM), but whether testosterone replacement therapy (TRT) produces clinically relevant metabolic benefits remains uncertain. This systematic review evaluated double-blind randomized placebo-controlled trials of TRT in obese men with functional secondary hypogonadism and either T2DM or metabolic syndrome. Methods: PubMed and Scopus were searched up to May 2026. Eligible studies enrolled adult men with biochemical hypogonadism and T2DM or metabolic syndrome, compared TRT with placebo, and reported metabolic outcomes. Open-label, single-blind, and non-randomized studies were excluded. Risk of bias was assessed with the Cochrane RoB 2 framework and certainty of evidence with GRADE. Results: Eight trials were included. The most consistent signal was improvement in body composition, mainly increased lean mass and reduced fat mass. Effects on insulin resistance were favorable in several trials but heterogeneous. GRADE certainty was low for fat mass, lean mass, and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR), and it was very low for clamp-derived insulin sensitivity, HbA1c, fasting glucose, lipid outcomes, inflammatory and vascular surrogate markers, and long-term safety. Conclusions: Low-certainty evidence suggests that TRT may improve body composition and HOMA-IR in carefully selected men with functional secondary hypogonadism and metabolic disease, whereas effects on glycemic and lipid outcomes remain very uncertain. TRT should not be regarded as an antidiabetic or lipid-lowering intervention. Larger phenotype-specific trials with longer follow-up are needed.