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Issues for surgery

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

Advice in the perioperative period

Elective surgery 

Continue.

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

Emergency surgery 

Continue.

Post-operative advice

Patients undergoing thyroidectomy

Patients undergoing thyroidectomy should stop antithyroid treatment post-operatively and start thyroid hormone replacement.

Other surgery

Continue usual dose.

Interactions with common anaesthetic agents

None.

Interactions with other common medicines used in the perioperative period

Antimicrobials

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

Myelosuppression

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

Further information

Carbimazole Important Safety Information

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.

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

Baxter K, Preston CL (eds), Stockley’s Drug Interactions (online) London: Pharmaceutical Press. http://www.medicinescomplete.com [Accessed on 30th June 2019]

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]

Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press. http://www.medicinescomplete.com [Accessed on 26th July 2019]

Palace MR. Perioperative Management of Thyroid Dysfunction. Health Services Insights. 2017; 10: 1-5

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]