MRCP-0425

MRCP-0425

A 30-year-old woman who works in a pharmacy comes to the clinic for review. Over the past few months, she has lost increasing amounts of weight and has become increasingly anxious about palpitations, which occur mostly at night. Her TSH is <0.1 IU/l (0.5-4.5). On examination, her BP is 122/72 mmHg, her pulse is 92 and regular. You cannot palpate a goitre or any nodules on examination of her neck. Which of the following investigations can differentiate between self-administration of thyroid hormone and endogenous causes of thyrotoxicosis?