Ciprofloxacin interacts with thyroid replacement therapy.

Case 1 An 80 year old woman with advanced thyroid cancer had maintained suppressed concentrations of thyroid stimulating hormone and stable free thyroxine by taking 125 g levothyroxine daily. She was admitted with a pathological fracture of the femur and complicating osteomyelitis. After four weeks’ treatment with oral ciprofloxacin (750 mg twice a day), intravenous dicloxacillin, and subcutaneous heparin, she complained of increasing tiredness. Her thyroid stimulating hormone concentration had increased to 44 mIU/l (reference range 0.4-4.4 mIU/l), free thyroxine had fallen to 4 pmol/l (12-22 pmol/l), and free triiodothyronine was 1.0 pmol/l (3.1-6.3 pmol/l). Increasing levothyroxine to 200 g daily had no effect. We reduced levothyroxine to 125 g daily and stopped ciprofloxacin, and thyroid function tests rapidly became normal. Other drugs (alfacalcidiol, propranolol, ranitidine, furosemide, methenamine hippurate, paracetamol, morphine, and ondansetron) were unchanged. We continued to give dicloxacillin and heparin as thyroid function returned to normal (figure). The patient died of metastatic thyroid cancer three weeks after discharge.