Methimazole Discontinuation Prior to Thyroid Uptake Scan
Yes, methimazole must be stopped before a thyroid uptake scan, with discontinuation for 24-48 hours being sufficient to obtain accurate uptake measurements while minimizing the risk of worsening hyperthyroidism.
Rationale for Discontinuation
Methimazole interferes with thyroid hormone synthesis by inhibiting the organification of iodine in the thyroid gland 1. This mechanism of action directly impacts radioiodine uptake measurements, as the drug blocks the same enzymatic processes that incorporate radioactive iodine into thyroid tissue during diagnostic scanning. Continuing methimazole during uptake scanning would artificially suppress the measured uptake values, leading to inaccurate assessment of thyroid function and potentially incorrect treatment planning.
Optimal Timing of Discontinuation
The evidence supports that 24-48 hours of methimazole discontinuation is adequate for diagnostic thyroid uptake scans:
A randomized clinical trial in 151 patients with Graves' disease demonstrated no significant difference in radioiodine uptake or treatment response between patients who discontinued methimazole for 24-48 hours versus those who stopped for longer periods (48-72 hours or 72-168 hours) 2.
Shorter discontinuation periods (24-48 hours) are preferable because they minimize the duration of uncontrolled hyperthyroidism while still allowing accurate uptake measurements 2.
The pharmacodynamic studies show that methimazole's inhibitory effect on thyroid hormone synthesis persists for approximately 24 hours after a single dose, with perchlorate discharge tests demonstrating residual activity at 24 hours 3.
Clinical Management During Discontinuation
Symptomatic control during the brief methimazole interruption:
Beta-blockers (such as propranolol or atenolol) should be continued or initiated to manage hyperthyroid symptoms during the drug holiday 4.
Patients should be counseled that this brief interruption is necessary for accurate diagnostic testing and that symptoms may temporarily worsen 2.
For patients with severe hyperthyroidism (grade 3-4 symptoms), consider whether the diagnostic scan can be deferred until better control is achieved, or proceed with close monitoring and aggressive symptomatic management 4.
Important Caveats
Distinguish between diagnostic uptake scans and therapeutic radioiodine:
The 24-48 hour discontinuation applies to diagnostic thyroid uptake scans used to assess thyroid function and uptake patterns.
For therapeutic radioiodine ablation, some practitioners use longer discontinuation periods (3-7 days), though research suggests this may not be necessary 2.
When methimazole is restarted after therapeutic radioiodine, be aware that thyroid hormone levels may transiently increase due to radiation-induced thyroiditis, particularly if the drug was discontinued for several days 5.
Pregnancy considerations:
In pregnant patients requiring thyroid uptake scanning (which should be avoided when possible), propylthiouracil is preferred in the first trimester due to methimazole's association with aplasia cutis congenita 6.
Radioactive iodine studies are absolutely contraindicated in pregnancy 4.
Resumption of Therapy
Methimazole should be restarted immediately after the uptake scan is completed to minimize the period of uncontrolled hyperthyroidism 2. The same dose can typically be continued unless thyroid function has changed significantly during the brief interruption.