A 48-year-old man presents with recurrent headaches and ahritic pain in his knees of 9 months in duration. He notes that his head size has recently increased. Physical examination reveals a blood pressure of 170/100 mm Hg. The patient is observed to have coarse facial features and a goiter. Urinalysis reveals glucosuria and hypercalciuria. Which of the following is the most likely explanation for this patient's clinical presentation?