Hyperpigmentation in the skin folds

A 62 year old man presented with a one year history of asymptomatic hyperpigmentation of the flexural and intertriginous areas, especially in the groin (fig 1). He had thickened skin on his palms, feet, and neck, had lost 15 kg in three years, and had experienced nail dystrophy (melanonychia and onychoatrophy) for two years. He had recently received a diagnosis of unexplained hypothyroidism and hypogonadism. What is the most likely diagnosis?