Testosterone deficiency, also known as hypogonadism, occurs when the body produces insufficient testosterone, the primary male sex hormone essential for energy, mood, muscle mass, bone strength, and sexual health. Symptoms may include fatigue, reduced libido, erectile dysfunction, loss of muscle, weight gain, depression, and cognitive decline. Causes include ageing, obesity, chronic illness, stress, and testicular or pituitary dysfunction. Left untreated, it increases risks of osteoporosis, cardiovascular problems, and metabolic syndrome. Diagnosis is confirmed through blood tests, and treatment may involve lifestyle changes, addressing root causes, or testosterone replacement therapy to restore vitality and wellbeing.