Background: The main cause of hyperthyroidism is diffuse toxic goiter (Graves’ disease), and the treatment of choice after
medical therapy failure is radioiodine (I-131). There are two common methods of determining the optimal I-131 dose:
calculated dose or fixed dose. The calculated dose method is based on the following formula: 75-200 microcuri/gram of
thyroid gland divided by the percentage of radioiodine uptake at 24 hours (24-hour RAIU). As this is quite complex, some
centers use fixed doses, such as 5, 10 or 15 mCi because it is simpler. At Rajavithi Hospital, the applied dose of I-131 is
determined based on the thyroid gland weight assessed by palpation and other clinical factors.
Objective: To study the mean I-131 dose for the initial treatment of hyperthyroidism in Rajavithi Hospital, to find the clinical factors that correlate with I-131 treatment dose, and to devise a formula to predict the optimal I-131 treatment dose.
Material and Method: This was a retrospective study of 510 patients with a diagnosis of hyperthyroidism who received initial
I-131 treatment at the Department of Nuclear Medicine in Rajavithi Hospital between January 2014 and June 2015. Baseline characteristics including age, sex, age at diagnosis, duration of antithyroid drug (ATD) therapy, gland weight (g), 3-hour RAIU and I-131 treatment dose were reviewed from medical records.
Results: The mean age + SD was 41.93+14.11 years (range 14-81 years), and the male to female ratio was 4.1:1. The mean
duration of ATD therapy was 3.54+4.02 years (min-max, 0.8-40.6 years). The mean gland weight was 54.35+32.95 grams,
and the mean 3-hour RAIU was 55.5+23.69%. The mean I-131 treatment dose was 14.84+5.71 mCi (min-max, 7-30 mCi).
There was no significant correlation between dose and age, age at diagnosis, duration of ATD therapy or 3-hour RAIU. The
study showed a significant correlation between I-131 dose and gland size, r= 0.938 (p<0.001), and the regression relationship
equation was: I-131 dose = 0.235 gland size, r = 0.938.
Conclusion: I-131 is the treatment of choice for hyperthyroidism after medical therapy failure, and there are various techniques for determining the optimal dose. At Rajavithi Hospital, the I-131 dose (mean = 14.84+5.71 mCi) is estimated based on the gland weight by palpation and other additional clinical factors. The present study provided a practical formula which is simple and practical for use in determining the I-131 dose for the treatment of hyperthyroidism: Dose of I-131 (mCi) = 0.235 x gland size (g).

Keywords: Hyperthyroidism, Radioiodine (I-131), 3-hour radioiodine uptake (3-hr RAIU)