Endocrine and Metaboliccommon
A 31-year-old woman presents with six months of fatigue, nausea and 7 kg of weight loss. She has been adding extra salt to everything and craves potato chips. Friends have commented that she looks tanned although she has spent the winter indoors. On examination there are brownish patches on the inside of her cheeks and darkening of an old appendicectomy scar. Blood pressure is 96/62 mmHg lying and 78/50 mmHg standing. Results: sodium 128 mmol/L (135–145), potassium 5.7 mmol/L (3.5–5.0), urea 8.9 mmol/L, glucose 3.4 mmol/L, calcium 2.62 mmol/L (2.10–2.55), TSH 6.2 mU/L (0.4–4.0).
Which one of the following is the most appropriate initial investigation?