MRCP2-4290

MRCP2-4290

A 57-year-old woman with a history of asthma attends her respiratory clinic appointment. She has been hospitalized twice in the past year due to exacerbations and also has hypothyroidism and angina. Her current medications include tiotropium, fluticasone, levothyroxine, simvastatin, aspirin, and GTN spray. During her last appointment, she was prescribed theophylline at a standard dose and completed a course of ciprofloxacin for a mild exacerbation. She is a smoker and has allergies to pollen.

Upon examination, she presents with a dry cough but is able to speak in full sentences. Her chest shows scattered wheezing throughout.

Lab results show a Na+ level of 145 mmol/l, K+ level of 3.9 mmol/l, urea level of 4.2 mmol/l, creatinine level of 56 µmol/l, and a theophylline level of 6 mcg/ml (normal range 10-20). Additionally, her chest x-ray shows hyperexpanded lung fields and her TSH level is 5.2 mU/l.

What could be the possible reason for her subtherapeutic theophylline level?