Endocrine Abstracts

Introduction: Several studies identified pre-treatment parameters that could help in the prediction of the thyroid functional outcome after radioactive iodine therapy (RAI) in Graves’ disease. However, the role of the thyroid peroxidase antibody (TPO Ab) status is not well studied.

Objectives: We investigated if a positive TPO Ab status before radioactive iodine (RAI) therapy in patients with Graves’ hyperthyroidism is a predictive factor for developing hypothyroidism post RAI.

Methods: We performed a retrospective study of patients with Graves’ hyperthyroidism with known TPO Ab status, receiving a first administration of RAI. Patients from four thyroid outpatient centres in Belgium receiving a first RAI therapy between the years 2011 and 2019 were studied. Clinical, laboratory, imaging, and treatment data were recorded from medical charts. Hypothyroidism and cure (defined as combined hypo- and euthyroidism) were evaluated in period 1 (≥2 and ≤9 months, closest to 6 months post RAI) and period 2 (>9 months and ≤ 24 months post RAI, closest to 12 months post RAI).

Results: One hundred fifty-two patients were included of which 105 (69%) were TPO Ab positive. Compared to TPO Ab negative patients, TPO Ab positive patients were younger, had a larger thyroid gland, and had more previous episodes of hyperthyroidism. There was no difference in THS receptor antibody titer at diagnosis, pre-treatment with anti-thyroid drugs or administered RAI activity. In period 1, 89% of the TPO Ab positive group developed hypothyroidism vs 72% in the TPO Ab negative group (P = 0.007). In period 2, the observation was similar: 88% vs. 72% (P = 0.019). Cure rate was similar in both groups in period 1 (91% vs. 85%, P = 0.238) and in period 2 (94% vs 86%, P = 0.146).

Conclusion: A positive TPO Ab status in patients with Graves’ hyperthyroidism receiving a first administration of RAI is associated with a higher risk of early hypothyroidism. The underlying mechanism warrants further investigation. Future studies investigating pre-treatment parameters affecting outcome after RAI in patients with Graves’ disease should incorporate TPO Ab status as a variable.