When to Request Repeat TSH After RAI Therapy
Repeat TSH testing should be performed 6-18 months after RAI therapy to assess treatment response and guide subsequent management, with the specific timing and frequency thereafter determined by the initial risk stratification and treatment response classification. 1
Initial Post-RAI Assessment Timeline
The first comprehensive assessment occurs at 6-18 months post-RAI, including TSH, thyroglobulin (Tg), thyroglobulin antibodies (TgAb), and neck ultrasound to classify treatment response. 1
Free T4 and TSH should be measured 2-3 months after RAI to assess adequacy of levothyroxine suppressive therapy and allow dose adjustment before the critical 6-12 month follow-up. 2
Subsequent TSH Monitoring Based on Treatment Response
Excellent Response (Undetectable Tg, Negative Imaging)
TSH and Tg/TgAb every 12-24 months for low-risk patients with excellent response, maintaining TSH at 0.5-2.0 mIU/ml. 1, 3
TSH and Tg/TgAb every 6-12 months for high-risk patients even with excellent response, as undetectable Tg may reflect tumor dedifferentiation rather than absence of disease. 1
Biochemical Incomplete or Indeterminate Response (Detectable Tg, Negative Imaging)
TSH and Tg/TgAb every 6-12 months for intermediate-risk patients with biochemical incomplete response, maintaining TSH at 0.1-0.5 mIU/ml. 1, 4
TSH and Tg/TgAb every 3-6 months for patients with rising Tg trends or rising TgAb levels, as short Tg doubling time (<1 year) is associated with poor outcomes. 1
Structural Incomplete Response (Visible Disease on Imaging)
- TSH and Tg/TgAb every 3-6 months with concurrent imaging surveillance, maintaining TSH <0.1 mIU/ml regardless of initial risk classification. 1, 4
Critical Timing Considerations
Do not assess treatment failure at 6 months for Graves' disease, as 26.9% of patients develop hypothyroidism by 3 months, 62.5% by 6 months, and 77.9% by 12 months—with 20.2% remaining hyperthyroid at 6 months but only 3.8% at 12 months. 5
Transient hypothyroidism can occur within 3-6 months post-RAI and may resolve, leading to euthyroidism or persistent hyperthyroidism requiring repeat RAI—failure to recognize this pattern delays optimal treatment. 6
Common Pitfalls to Avoid
Never interpret isolated Tg measurements when residual thyroid tissue is present—almost 60% of patients post-total thyroidectomy without RAI have basal Tg ≥0.2 ng/ml due to normal residual tissue, requiring trend monitoring rather than single values. 1, 3
Do not maintain aggressive TSH suppression (<0.1 mIU/ml) indefinitely in patients with excellent response, as this increases risks of atrial fibrillation, bone loss, and cardiovascular events without improving outcomes in low-risk disease. 3, 4
Avoid premature repeat RAI dosing—patients with high 2-hour iodine uptake values and those requiring antithyroid drugs post-RAI are more likely to develop late-onset hypothyroidism (>6 months), making 12-month assessment more appropriate than 6-month evaluation. 5