Should I discontinue methimazole (antithyroid medication) due to high Thyroid-Stimulating Hormone (TSH) levels?

Management of High TSH in Patients on Methimazole

Methimazole should be discontinued if TSH levels are elevated, as this indicates hypothyroidism which is a known side effect of the medication requiring dose adjustment or discontinuation. 1

Assessment of Elevated TSH on Methimazole

• Elevated TSH with normal or low free T4 while on methimazole indicates that the medication has caused hypothyroidism, which requires immediate attention 2
• Methimazole can cause hypothyroidism necessitating routine monitoring of TSH and free T4 levels with adjustments in dosing to maintain a euthyroid state 1
• Drawing both TSH and FT4 is especially important when patients are symptomatic and hypothyroidism is suspected 2

Management Algorithm

1. For patients with elevated TSH (>4.5 mIU/L) on methimazole:

   • If asymptomatic with TSH between 4.5-10 mIU/L: Consider dose reduction 2
   • If TSH >10 mIU/L or symptomatic: Discontinue or significantly reduce methimazole dose 2

2. After discontinuation or dose adjustment:

   • Monitor thyroid function every 4-6 weeks initially 3
   • Check both TSH and FT4 to properly assess thyroid status 2

Special Considerations

• Elevated TSH during methimazole therapy may actually be a favorable prognostic indicator for long-term remission in Graves' disease patients 4
• In a study of patients with Graves' disease, those who developed elevated TSH (>10 μIU/mL) during methimazole therapy had significantly higher remission rates at 24 months after discontinuation (85.0% vs 54.1%) 4
• Long-term low-dose methimazole may be considered for patients with Graves' disease who have completed standard treatment courses but are at high risk for relapse 5

Monitoring After Discontinuation

• Close thyroid function monitoring is essential after methimazole discontinuation, with regular checks of TSH and FT4, especially during the first year 3
• The American Academy of Family Physicians recommends monitoring thyroid function every 1-3 months for the first year after discontinuation 3
• Be vigilant for signs of recurrent hyperthyroidism, which may occur in approximately 19-41% of patients within 3 years of discontinuation 6, 5

Common Pitfalls to Avoid

• Failing to check both TSH and FT4 when evaluating thyroid status - low TSH with low FT4 indicates central hypothyroidism requiring different management 2
• Overlooking the possibility that elevated TSH can be seen in the recovery phase of thyroiditis - in asymptomatic patients with normal FT4, monitoring for 3-4 weeks before treatment adjustment may be appropriate 2
• Not considering that hypothyroidism during methimazole treatment may actually predict better long-term outcomes in Graves' disease 4

Alternative Treatment Options

• If discontinuation of methimazole is not appropriate due to risk of recurrent hyperthyroidism, definitive treatment options such as radioactive iodine (I-131) therapy or thyroidectomy should be considered 3
• Some studies suggest that adding thyroxine to methimazole treatment may decrease antibody production and reduce recurrence rates in Graves' disease 7