A 65-year-old man presents to the neurology clinic with complaints of excessive daytime drowsiness despite good sleep patterns. His wife reports that he appears vague and drowsy at times during the day, with fluctuating attention span. He has also been experiencing visual hallucinations and has become increasingly slow at dressing and walking. His medical history includes hypertension treated with bendroflumethiazide and a previous left-sided cerebrovascular accident with good recovery. He is a smoker and drinks 10 units of alcohol per week. On examination, he has a small goitre and hypophonia with an expressionless face. His mini mental state examination reveals deficiencies in verbal fluency, executive function, and visuospatial testing. Peripheral nervous system examination reveals bradykinesia, increased tone bilaterally, and unilaterally brisk reflexes on the left with delayed relaxation on the right. The left plantar response is extensor. Thyroxine levels are low, and thyroid stimulating hormone levels are high. What is the best next step in managing this patient?