Intended for healthcare professionals


Ciprofloxacin interacts with thyroid replacement therapy

BMJ 2005; 330 doi: (Published 28 April 2005) Cite this as: BMJ 2005;330:1002
  1. John G Cooper, endocrinologist (john.cooper{at},
  2. Knut Harboe, senior house officer2,
  3. Sofia K Frost, pharmacist4,
  4. Øyvind Skadberg, consultant physician3
  1. 1 Department of Medicine, Stavanger University Hospital, PO Box 8100, 4068 Stavanger, Norway,
  2. 2 Department of Orthopaedic Surgery, Stavanger University Hospital,
  3. 3 Department of Medical Biochemistry, Stavanger University Hospital,
  4. 4 Regional Drug Information Centre, Haukeland University Hospital, 5021 Bergen, Norway
  1. Correspondence to: J Cooper
  • Accepted 1 February 2005


We report two cases of unexplained hypothyroidism in patients taking oral ciprofloxacin (figure). Nursing staff gave levothyroxine. Prescription charts showed no drug administration errors. Patients did not have nausea, vomiting, or diarrhoea.


Unexplained hypothyroidism in patients taking oral ciprofloxacin

Case reports

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 μg 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 μg daily had no effect. We reduced levothyroxine to 125 μg 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.

Case 2

A 79 year old woman with rheumatoid arthritis, manic depression, cardiac failure, chronic obstructive airways disease, and hypothyroidism was admitted with a wound infection after a transfemoral amputation. She had maintained stable thyroid function tests on a daily dose of 150 μg levothyroxine. After three weeks' treatment with oral ciprofloxacin (500 mg twice a day), her concentration of thyroid stimulating hormone had increased from 1.6 to 19 mIU/l and free thyroxine had fallen from 22 to 13 pmol/l. Switching from concomitant administration of levothyroxine and ciprofloxacin to administering the drugs with a six hour gap resulted in rapid normalisation of the thyroid function tests (figure). Other drugs (zuclopenthixol, enalapril, bumetanide, prednisolone, folic acid, lactulose, acetylcysteine, hydroxychloroquine, paracetamol, ipratropium bromide, salbutamol, nystatin) remained unchanged.


Oral ciprofloxacin may interact with levothyroxine if they are given together. The most plausible explanation is that ciprofloxacin decreases the absorption of levothyroxine. Antacids, laxatives, colestipol, colestyramine, ferrous sulphate, sulcralfate, and raloxifene have been reported to decrease the absorption of levothyroxine,13 but we have not found any previous reports of an interaction between levothyroxine and ciprofloxacin. Neither have the WHO Collaborating Centre for International Drug Monitoring nor the manufacturer of ciprofloxacin. This interaction is important for patients taking long courses of ciprofloxacin. It is prudent to advise patients to take levothyroxine and other drugs at different times.


  • Funding None.

  • Competing interests None declared.


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