What monitoring of blood parameters is required for a patient on methimazole (for carbimazole) therapy?

From the Guidelines

Regular blood monitoring is crucial for patients on methimazole (or carbimazole) to prevent and detect potential adverse effects, and it should be performed before starting treatment, then every 2-4 weeks during the first 3 months of therapy, and periodically thereafter. Monitoring of bloods whilst a patient is on methimazole for carbimazole should include complete blood counts (CBC) and liver function tests (LFTs) to detect potential side effects such as agranulocytosis and hepatotoxicity, as suggested by 1. Thyroid function tests (TFTs), including TSH, free T4, and sometimes free T3, should be checked every 4-6 weeks initially until the patient is euthyroid, then every 2-3 months once stable, as this allows for dose adjustments to maintain euthyroidism using the lowest effective dose to minimize side effects. Key points to consider when monitoring bloods in patients on methimazole include:

• Performing CBC and LFTs before starting treatment, then every 2-4 weeks during the first 3 months of therapy, and periodically thereafter (typically every 2-3 months) for the duration of treatment, as recommended by 1.
• Checking TFTs every 4-6 weeks initially until the patient is euthyroid, then every 2-3 months once stable.
• Educating patients to report symptoms such as fever, sore throat, mouth ulcers (suggesting neutropenia), jaundice, dark urine, or right upper quadrant pain (suggesting liver injury) immediately, as these may necessitate urgent blood tests regardless of the regular monitoring schedule. Although the study by 1 provides recommendations for toxicity monitoring in the context of azathioprine, its findings on the importance of regular monitoring of liver blood tests and full blood counts (FBC) can be applied to the monitoring of patients on methimazole, emphasizing the need for regular checks to ensure patient safety.

From the FDA Drug Label

Patients who receive methimazole should be under close surveillance and should be cautioned to report immediately any evidence of illness, particularly sore throat, skin eruptions, fever, headache, or general malaise. In such cases, white-blood-cell and differential counts should be obtained to determine whether agranulocytosis has developed Laboratory Tests Because methimazole may cause hypoprothrombinemia and bleeding, prothrombin time should be monitored during therapy with the drug, especially before surgical procedures. Thyroid function tests should be monitored periodically during therapy Once clinical evidence of hyperthyroidism has resolved, the finding of a rising serum TSH indicates that a lower maintenance dose of methimazole should be employed.

The patient on methimazole for carbimazole should be monitored for:

• Agranulocytosis: White-blood-cell and differential counts should be obtained if symptoms such as sore throat, fever, or general malaise occur.
• Hypoprothrombinemia and bleeding: Prothrombin time should be monitored during therapy, especially before surgical procedures.
• Thyroid function: Thyroid function tests should be monitored periodically during therapy to adjust the dose and maintain a euthyroid state.
• Liver function: Liver function tests, such as bilirubin, alkaline phosphatase, ALT, and AST, should be monitored if symptoms suggestive of hepatic dysfunction occur 2. It is essential to monitor these parameters to minimize the risk of adverse reactions and ensure the safe use of methimazole 2.

From the Research

Monitoring of Bloods Whilst Patient on Methimazole for Carbimazole

• The antithyroid drugs, including methimazole, can cause dose-dependent hypothyroidism, and thus thyroid function should be repeatedly checked while the patient is on antithyroid drugs 3.
• Methimazole treatment can induce insignificant liver function test (LFT) elevation, <2× the upper limit of normal (ULN), and can be safely administered in hyperthyroid patients with abnormal LFT 4.
• Methimazole has been associated with hepatotoxicity, including severe cholestasis, and older age of the patient and higher dose of the drug are risk factors for cholestatic injury 5.
• Monitoring of LFTs is recommended during methimazole treatment, as hepatotoxicity can occur, particularly in children, and the use of methimazole should be avoided in children due to the risk of severe hepatic failure 6.
• In addition to LFTs, monitoring for other potential side effects of methimazole, such as neutropenia, agranulocytosis, and arthralgias, is also recommended 3.