Antithyroid Medication


 Carbimazole, Propylthiouracil

  Issues for Surgery


None – providing patient’s hyperthyroidism is well controlled (see Further Information).


  Advice in the Perioperative period


Elective Surgery

Continue.

Propylthiouracil may cause thrombocytopenia hence patients undergoing surgery should have their prothrombin time checked pre-operatively1.

A thyroid stimulating hormone (TSH) test should be included in pre-operative assessment to determine the adequacy of treatment and to ensure that thyroid therapy is optimised before surgery2.

Emergency Surgery

Continue.

Post-operative Advice

Patients undergoing Thyroidectomy
Patients undergoing thyroidectomy should stop antithyroid treatment post-operatively and start thyroid hormone replacement (see Thyroid Hormones monograph).

Other surgery

Continue usual dose.


  Interaction(s) with Common Anaesthetic Agents


None1, 3, 4, 5, 6, 7.


  Interaction(s) with other Common Medicines used in the Perioperative Period


Antimicrobials

Carbimazole may inhibit the metabolism of erythromycin, leading to reduced clearance of erythromycinand possibly torsades de pointes in susceptible individuals5.

Myelosuppression
Carbimazole and propylthiouracil can both cause myelosuppression. Concomitant treatment with linezolid, sulfamethoxazole (found in co-trimoxazole) or trimethoprim can increase the risk of myelosuppression3.


  Further Information


Carbimazole Important Safety Information3

Healthcare professionals should be alert to the potential for carbimazole-induced bone marrow suppression and the need to stop treatment promptly: -

  • Patients should be asked to report symptoms and signs suggestive of infection, especially sore throat.
  • A white blood cell count should be performed if there is any clinical evidence of infection.
  • Carbimazole should be stopped promptly if there is clinical or laboratory evidence of neutropenia. Urgent advice should be sought from a Haematologist in the case of neutropenia.

Propylthiouracil Safety Information

Hepatic Reactions
Severe hepatic reactions have been reported with propylthiouracil use, including fatal cases and cases requiring liver transplant. Monitor the patient for signs of liver disorder (e.g. anorexia, nausea, vomiting, fatigue, abdominal pain, jaundice, dark urine, or pruritus). Discontinue treatment immediately if significant liver-enzyme abnormalities develop1, 3.

Agranulocytosis
Monitor patients for signs of agranulocytosis (e.g. fever, sore throat, mouth ulcers, malaise, non-specific illness or other symptoms of infection). A full blood count (FBC) should be performed and treatment discontinued immediately if there is clinical or laboratory evidence of neutropenia1, 3. Urgent advice should be sought from a Haematologist in the case of neutropenia.

Thyrotoxicosis and Thyroid Storm

Thyrotoxicosis is a condition that occurs due to excessive circulating thyroid hormones (it includes hyperthyroidism). Signs and symptoms include weight loss, heat intolerance and palpitations.

Thyroid storm is a life-threatening form of untreated or under-managed hyperthyroidism. It is characterised by high fever, fast and irregular heart rate, elevated blood pressure, vomiting, diarrhoea and agitation.

There is a potential risk of thyrotoxicosis or thyroid storm if antithyroid hormones are omitted perioperatively. However, the actual risk is dependent on the patient’s TFTs and the length of time they are omitted. It takes 4 – 6 weeks from being euthyroid to the patient becoming thyrotoxic once the antithryroid agents are stopped. The emergence of thyroid storm would be rare. If there are any concerns regarding the omission or continuation of antithyroid medication during the perioperative period, the patient’s Endocrinologist should be consulted.

 

  References


  1. Summary of Product Characteristics – Prophylthiouracil 50mg Tablets. Wockhardt UK Ltd. Accessed via www.medicines.org.uk 26/07/2019 [date of revision of the text December 2015]
  2. Palace MR. Perioperative Management of Thyroid Dysfunction. Health Services Insights. 2017; 10: 1-5
  3. Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press. http://www.medicinescomplete.com [Accessed on 26th July 2019]
  4. Summary of Product Characteristics – Carbimazole 20mg tablets. ADVANZ. Accessed via www.medicines.org.uk 26/07/2019 [date of revision of the text June 2019]
  5. Baxter K, Preston CL (eds), Stockley’s Drug Interactions (online) London: Pharmaceutical Press. http://www.medicinescomplete.com [Accessed on 30th June 2019]
  6. Carbimazole. In: Brayfield A (Ed), Martindale: The Complete Drug Reference. London: The Royal Pharmaceutical Society of Great Britain. http://www.medicinescomplete.com [Accessed 26th July 2019]
  7. Propylthiouracil. In: Brayfield A (Ed), Martindale: The Complete Drug Reference. London: The Royal Pharmaceutical Society of Great Britain. http://www.medicinescomplete.com [Accessed 26th July 2019]