Outcome and incidence of hypothyroidism in low-dose radioactive iodine treatment for hyperthyroidism.

Objective: The study aimed to analyze the outcome of low-dose radioactive iodine (RAI) treatment for hyperthyroidism, disclose whether age and gender influence the outcome and determine the incidence and onset time of hypothyroidism following low-dose RAI.

Material and methods: A total of 158 patients who received doses less than 370 Mbq RAI were enrolled in the study. Treatment outcome and incidence of hypothyroidism were compared between different gender (45 male vs.113 female), age (77 patients ≥45 years old vs. 81 patients <45 years old) and dose (39 patients receiving higher doses RAI vs. 119 receiving lower dose with a cutoff of 222 MBq) groups. Treatment outcomes were categorized into post-treatment hypothyroidism, treatment failure (persistent hyperthyroidism), and euthyroidism. In those becoming hypothyroid, time to develop hypothyroidism was calculated for cumulative incidences over time.

Results: Out of 158 patients, 47 (29.7%) developed hypothyroidism, 101 (63.9%) had treatment failure, and 10 (6.3%) remained euthyroid after treatment. Response rates (33.6% vs. 43.5%, p = 0.260) and hypothyroidism incidences (26.9% vs. 38.5%, p = 0.170) did not differ significantly between lower and higher dose groups, neither between lower and higher age groups (p = 0.69 in response rates and p = 0.75 in hypothyroidism incidence). Females exhibited higher response rates (42.5% vs. 20.0%, p = 0.008) and hypothyroidism incidence (46.3% vs. 13.3%, p = 0.004) compared to males. Hypothyroidism onset occurred at a mean of 24.0 ± 29.2 months, and the cumulative incidences over time were 47% and 60% in six and twelve months, respectively.

Conclusions: Low-dose RAI has a low response rate for treating hyperthyroidism. Although there may be a lower incidence of hypothyroidism following low-dose RAI compared to high-dose RAI, hypothyroidism may occur early after treatment. Besides, females have higher response rates but more incidence of hypothyroidism. The balance between the risks and benefits of using low-dose RAI should be taken into deliberate consideration.