A 38-year-old man with schizophrenia has been on risperidone 6 mg/day for 8 months. He presents with complaints of galactorrhea, amenorrhea in his wife (who lives with him), and erectile dysfunction. His prolactin level is 85 ng/mL (normal <25 ng/mL). What is the investigation of choice to exclude secondary causes of hyperprolactinemia before attributing it to antipsychotic use?