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Carbimazole propylthiouracil (PTU) warning

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To the patient: warning- please read this now.

Carbimazole/PTU treatment and sore mouth/throat.

You have been started on carbimazole treatment for an overactive thyroid. This is a very safe treatment that has been used for over 50 years.

However, very rarely, a patient reacts to the drug with a sudden loss of white blood cells (‘neutrophils’) from the blood.

This puts he/she at very high risk from infections and the drug MUST BE STOPPED IMMEDIATELY. The first sign of this happening is A SEVERE SORE MOUTH OR THROAT FOR NO OBVIOUS REASON.

If you suspect this may have happened:

  •  Do NOT take any more doses of the tablet.
  • Contact an emergency doctor or a casualty department THE SAME DAY (even if it is a weekend) and show him this letter and or the tablets.

    To the doctor:

    This patient is on CARBIMAZOLE or PTU. If he/she has a sore throat or mouth it may indicate agranulocytosis, a very rare side effect.


    • Stop carbimazole/PTU immediately.
    • Check neutrophil count urgently, NOT just the total white cell count (which may be normal). If the neutrophil count is < 1.0 do NOT recommence treatment and admit patient for neutropaenia. If the neutrophil count is 1.0 - 2.0, measure it again the next day off treatment. If it is >2.0, it is safe to continue treatment.
    • Carbimazole/PTU and thyrotoxicosis also cause a mild reduction in white cell count which is of no significance. A skin rash on carbimazole/PTU is also common and does not indicate neutropaenia.

      Useful contact details

      Department of Endocrinology, Royal Sussex County Hospital, Brighton. Telephone: 01273 696955

      Princess Royal Hospital, Haywards Heath. Telephone: 01444 441881

      Please see here for a link to a short video about thyroid treatment on our website.

        This information is intended for patients receiving care in Brighton & Hove or Haywards Heath.

        The information in this leaflet is for guidance purposes only and is in no way intended to replace professional clinical advice by a qualified practitioner.

          Publication Date: July 2021

          Review Date: April 2024

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