A 37year-old male diagnosed with hypertension has been treated with a low sodium diet and hydrochlorothiazide (HCTZ) 50 mg qd for the past two months. He denies Family history of cardiovascular disease. At today's follow-up visit his BP=150/90 and T=I00 F. Physical examination reveals no bruits, clear chest, no atrial gallop, edema, and tenderness of the left ankle, and an intact neurological system. Which laboratory values will provide the most useful follow-up information?