A 24-year-old man presents to your clinic with a 6-month history of fatigue. On examination, he is pale. His BMI is 16, and his blood pressure is 92/58 mm Hg. Initial laboratory work shows the following:
Creatinine: 64 µmol/L (49–93)
Potassium: 3.0 mmol/L (3.5–5.1)
Sodium: 138 mmol/L (136–146)
TSH: 2.40 mIU/L (0.34–5.60)
CBC: Normal
Which one of the following is the best next step?
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