A 52-year-old man from Delhi presents with progressive darkening and thickening of skin over the neck, axillae, and groin for 6 months. On examination, he has velvety, hyperpigmented plaques with accentuated skin markings. He is obese (BMI 31 kg/m²) and has fasting blood glucose of 156 mg/dL. What is the most appropriate next step in management?