Can Tc 99m (Technetium-99m) thyroid scintigraphy be performed after starting carbimazole?

Tc-99m Thyroid Scintigraphy After Starting Carbimazole

Tc-99m thyroid scintigraphy should not be performed while a patient is taking carbimazole, as the medication should be discontinued for at least 2-3 days before the scan to avoid false results.

Effect of Carbimazole on Thyroid Scintigraphy

• Carbimazole interferes with the uptake of Tc-99m pertechnetate in the thyroid gland, which can lead to falsely decreased uptake values and potential misinterpretation of scan results 1, 2
• Tc-99m pertechnetate is a radiotracer that is taken up by the thyroid gland in a similar manner to iodine, but it is not organified (incorporated into thyroid hormones) 3
• Carbimazole inhibits thyroid hormone synthesis by blocking the organification of iodine, which can also affect the uptake of Tc-99m pertechnetate 1

Recommended Protocol

• Carbimazole should be discontinued for at least 2-3 days before performing Tc-99m thyroid scintigraphy to obtain accurate results 4
• Longer discontinuation periods (5-7 days) may be necessary for patients on high doses or long-term carbimazole therapy to ensure complete washout of the drug effect 1
• The timing of discontinuation should be balanced against the risk of worsening hyperthyroidism symptoms during the drug-free period 4

Clinical Implications

• Performing Tc-99m thyroid scintigraphy while the patient is on carbimazole can result in:

  • Falsely decreased uptake values 2
  • Potential misdiagnosis of the underlying thyroid condition 1
  • Inaccurate assessment of thyroid function and autonomy 1, 2

• Studies have shown that patients who undergo thyroid scintigraphy while on carbimazole may show suppressed thyroid tissue that would otherwise be visible after proper discontinuation of the medication 5

Alternative Approaches

• If carbimazole cannot be safely discontinued, consider:
  • Using Tc-99m-MIBI (methoxy isobutyl isonitrile) instead of Tc-99m pertechnetate, as its uptake appears to be less dependent on TSH control and may provide better visualization of suppressed thyroid tissue even in patients on antithyroid medications 5
  • Scheduling the scan before initiating carbimazole therapy if the clinical situation allows 2

Prognostic Value

• Properly performed Tc-99m thyroid scintigraphy after completing carbimazole therapy can provide valuable prognostic information:
  • Patients with persistently increased Tc-99m thyroid uptake after a full course of carbimazole treatment have a high likelihood of relapse 6
  • Patients whose Tc-99m thyroid uptake has reverted to normal after treatment are more likely to stay in long-term remission 6

Common Pitfalls to Avoid

• Failure to discontinue carbimazole before the scan is reported in approximately 25% of patients undergoing thyroid scintigraphy, leading to potential misinterpretation of results 2
• Inadequate patient education about the need to temporarily stop medication before the scan is a common issue that needs to be addressed 2
• The scan should be performed within 20 minutes of preparing the Tc-99m-pertechnetate to ensure optimal imaging quality, as only the lipophilic complex can freely cross cell membranes 3

Remember that accurate thyroid scintigraphy results are essential for proper management decisions in hyperthyroidism, including determining the need for radioactive iodine therapy or surgery.