MRCP2-1538

A 50-year-old woman presents with symptoms of tiredness, weight loss, anxiety, and difficulty sleeping. She had been receiving a combined cyclical oestrogen/progesterone hormone replacement therapy. Upon examination, she appeared thin with a pulse of 110 beats per minute, a fine tremor, and proximal myopathy. Her spleen tip was barely palpable. Initial investigations revealed elevated serum total thyroxine and low plasma TSH levels, as well as elevated alkaline phosphatase and gamma glutamyl transferase levels. The patient was started on carbimazole and propranolol, which resulted in a euthyroid state after six weeks. However, after a year of treatment, the patient reported increasing fatigue and was referred to the medical outpatient. Two weeks prior to her appointment, she developed a chest infection and was prescribed erythromycin. At the clinic, her alkaline phosphatase levels had decreased, but her thyroid function tests showed low serum total thyroxine and high plasma TSH levels. What is the cause of her elevated alkaline phosphatase?