A 33-year-old man complains of a tingling sensation in his hands for several months which occasionally awakens him during sleep. The patient has noticed he has gained weight and no longer wears his wedding ring as it has become too tight. You notice the patient is sweating while speaking to you and has quite a large jaw, furrowed tongue and large hands. His blood pressure reading is 142/91 mmHg. The most appropriate investigation would be: