A 55-year-old man presents to his General Practitioner with a 4-month history of tiredness. On examination, he is noted to have breast enlargement (gynaecomastia). Blood tests show: - TSH: 0.2 mIU/L (reference range: 0.4–4.3 mIU/L) - Free T4: 6 pmol/L (reference range: 9–23 pmol/L) What is the most likely diagnosis?