6.2.2.1 Antithyroid drugs

ChoiceDrugDosage
Formulary choice

Carbimazole tablets 5mg, 20mg
To be initiated on specialist advice

Dose:
20-60mg (BNF states 15-40mg) daily until euthyroid (usually 4-8 weeks), then progressively reduced to a maintenance dose, typically 5-15mg, daily usually for 12-18 months (specify duration of treatment)

Prescribing Notes

  • Refer to NICE guidance on Thyroid disease: assessment and management
  • Carbimazole should be initiated under specialist advice to ensure the correct diagnosis is made and that treatment is appropriate. Carbimazole can be given by titration method or in a block and replacement regimen.
  • Carbimazole is a finite treatment and guidance should be given to patients on the proposed treatment duration.
  • Carbimazole has rarely been associated with bone marrow suppression and treatment should be stopped promptly if there is clinical or laboratory evidence of neutropenia. 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.
  • Propylthiouracil may be an alternative for patients who suffer sensitivity reactions to carbimazole and similar advice regarding neutropenia should be given when using this drug
  • Severe hepatic reactions have been reported with propylthiouracil, including fatal cases and cases requiring liver transplant. Liver function should be monitored and propylthiouracil discontinued if significant liver-enzyme abnormalities develop.
  • All pregnant women with thyrotoxicosis should be seen by a specialist endocrine team early in gestation. Propylthiouracil is the drug of choice during early pregnancy and breastfeeding.

Cautions

  • Carbimazole is associated with an increased risk of congenital malformations, especially when administered in the first trimester of pregnancy and at high doses. Women of childbearing potential should use effective contraception during treatment with carbimazole. See MHRA for further details.
  • Cases of acute pancreatitis have been reported very infrequently during treatment with carbimazole. If acute pancreatitis occurs during treatment with carbimazole, immediately and permanently stop treatment. Re-exposure to carbimazole may result in life-threatening acute pancreatitis with a decreased time to onset. See MHRA for further details.