Endocrine Abstracts

Objective: We investigated if a positive thyroid peroxidase antibody (TPO Ab) status before radioactive iodine (RAI) therapy in patients with Graves’ hyperthyroidism is a predictive factor for developing hypothyroidism after 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 4 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. 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). In a multivariate logistic regression analysis, adjusting for age at diagnosis, fT4 at diagnosis, TSH-R Ab at diagnosis, thyroid volume at diagnosis, ATD preceding RAI and RAI activity, the adjusted OR was 4.16 (95% CI: 1.0–18.83; P=0.052) in period 1 and 4.78 (95% CI: 1.27–20.18; P=0.024) in period 2.

Conclusion: To date, the role of the TPO Ab status in patients with Graves’ hyperthyroidism has not been well studied as a predictive parameter for thyroid functional outcome after first administration of RAI. We show that TPO Ab-positive patients were more likely to develop early hypothyroidism after the first administration of RAI, regardless of previously established factors associated with cure or treatment failure after RAI. Future studies investigating pre-treatment parameters affecting the outcome after RAI in patients with Graves’ disease should incorporate TPO Ab status as a variable.