Can testosterone injections cause weight gain in adults with hypogonadism (low sex hormone levels)?

Testosterone Injections and Weight Changes in Hypogonadism

Testosterone injections in men with hypogonadism cause weight LOSS, not weight gain—specifically, they produce progressive and sustained reductions in body weight, waist circumference, and body fat while increasing lean body mass. 1, 2, 3

Expected Body Composition Changes with Testosterone Therapy

Weight Loss and Fat Reduction:

• Long-term testosterone therapy (6-8 years) produces significant weight loss across all obesity classes, with reductions ranging from 17-30 kg depending on baseline obesity severity 2
• In men with class I obesity (BMI 30-34.9), weight decreased by an average of 17.4 kg (-16.8%) over 8 years 2
• In men with class II obesity (BMI 35-39.9), weight decreased by 25.3 kg (-21.5%) 2
• In men with class III obesity (BMI ≥40), weight decreased by 30.5 kg (-23.6%) 2
• Waist circumference decreased by 10.6-14.3 cm across all obesity classes 2

Lean Body Mass Changes:

• Testosterone therapy increases lean body mass while simultaneously decreasing body fat 1, 4
• The net effect is typically weight loss because fat loss exceeds muscle gain in most hypogonadal men 1

Metabolic Improvements Beyond Weight

Cardiometabolic Benefits:

• Improvements in fasting plasma glucose and insulin resistance (HOMA-IR) 1
• Reductions in HbA1c levels 3
• Improved lipid profiles with increased HDL cholesterol and decreased triglycerides 1, 3
• Gradual decreases in both systolic and diastolic blood pressure 3

Mechanism of Weight Loss

Physiological Basis:

• Testosterone deficiency in males is associated with increased abdominal fat mass and reduced lean body mass 1
• In obese men, increased aromatization of testosterone to estradiol in adipose tissue creates a vicious cycle of further testosterone suppression 1
• Testosterone replacement breaks this cycle, improving energy balance and insulin sensitivity 1

Important Clinical Caveats

Fluid Retention vs. Fat Loss:

• While testosterone can cause sodium and water retention initially 5, this is typically transient and does not offset the substantial fat loss that occurs with long-term therapy
• The FDA label notes that androgens "cause retention of nitrogen, sodium, potassium, and phosphorous" 5, but clinical studies demonstrate net weight loss over time 2, 3

Time Course:

• Weight loss is progressive and sustained over years, not immediate 2, 3
• Maximum benefits require long-term therapy (5-8 years) with consistent treatment 2, 3

Prerequisite for Treatment:

• Weight loss only occurs in men with confirmed biochemical hypogonadism (morning testosterone <300 ng/dL on two separate occasions) 1, 6
• Testosterone therapy in eugonadal men (normal testosterone levels) is contraindicated and should never be used for weight loss 6

Clinical Bottom Line

For men with documented hypogonadism, testosterone injections cause substantial weight loss through fat reduction and improved body composition, not weight gain. The concern about "gaining weight" on testosterone is unfounded in hypogonadal men—the opposite occurs with appropriate long-term therapy. 2, 3