Excessive dietary iodine is known to predispose individuals to Graves’ disease (GD), and patients are accordingly advised to limit iodine intake. However, the impact of iodine restriction during antithyroid drug (ATD) therapy remains uncertain. This study aimed to evaluate iodine status in patients with GD and its effect on disease control.

In this prospective cohort (2019–2024), patients with newly diagnosed GD at Taipei Veterans General Hospital were followed at 2 weeks and at 1, 3, 6, and 12 months. Clinical parameters, thyroid function, ultrasonography, and autoantibodies were monitored, and patients were instructed to avoid iodine-rich foods. At diagnosis, 14 consecutive daily spot urine samples were collected and compared with samples from GD patients in remission and from healthy controls. Urinary iodine concentration (UIC) was measured using inductively coupled plasma mass spectrometry, with patients stratified according to UIC levels. Logistic regression analyses were performed to identify predictors of failure to achieve meaningful antibody reduction.

A total of 55 newly diagnosed GD patients were enrolled (74.6% female; mean age 45.3 ± 13.0 years). Median and mean UICs were significantly higher in newly diagnosed patients than in remission cases (n = 56) and healthy controls (n = 108; both P < 0.001). After ATD treatment, euthyroidism was achieved in 76.4%, 79.2%, and 85.4% of patients at 3, 6, and 12 months, respectively. At 3 months, 51.1% of patients demonstrated ≥50% reduction in thyroid-stimulating immunoglobulin (TSI) or thyrotropin-binding inhibiting immunoglobulin (TBII), increasing to 79.1% and 88.2% at 6 and 12 months. Patients with baseline mean UIC <200 μg/L had significantly lower free T4 at 2 weeks (2.0 ± 0.8 vs. 2.6 ± 1.3 ng/dL, P = 0.039) and greater antibody decline at 1 month (−18.0 ± 30.6 vs. 28.4 ± 6.9, P = 0.029). In multivariate logistic analysis, larger thyroid volume and higher median UIC independently predicted failure to achieve ≥50% antibody reduction at 6 months.

Elevated UIC was associated with slower biochemical response and reduced antibody decline, suggesting that iodine excess may contribute to treatment resistance. Moderate iodine restriction during the initial phase of ATD therapy appears beneficial.