Changes in thyroid stimulating hormone receptor antibody and thyroid weight following a single radioiodine therapy in hyperthyroidism over 9 years

Objectives: Radioactive iodine I-131 therapy (RAI) is a definitive treatment in hyperthyroidism, However, less is known regarding long-term changes in thyroid stimulating hormone receptor antibody (TRAb) and thyroid weight (TW) after RAI. We sequentially evaluated changes in TRAb and TW during the 9-years follow up after RAI.

Methods: We enrolled 39 patients with biochemically confirmed Graves’ hyperthyroidism (M/F5/32, age:31-52yrs.) who underwent RAI one time and were followed up more than 9 years without an additional RAI. TRAb was measured by radioreceptor assay (normal range: -15.0-15.0%).Average of pretreatment value was 53.0%(13.2-87.7).TW was determine using ultrasound or CT. Average of pretreatment value was 56.2g(22.1-134.9). After counselling with a dietician, patients followed a low iodine diet (less than 300μg/day) from 7 days before to 3 days after RAI. All patients were pretreated by anti-thyroid drugs. ATDs were discontinued 3-4 days prior to RAI. Dose of I-131 was adjusted so that more than 100Gy was delivered to the thyroid gland. Otherwise, 555 MBq of I-131 was administered. Along with thyroid function, TRAb and TW were monitored every 6-12 months for 9 years. Additionally, patients were encouraged to continue regular visit to ophthalmologist to check the status of thyroid associated ophthalmopathy (TAO).

Results: Of 39 patients, euthyroidism was achieved and maintained in 2 (5%) and persistent hyperthyroidism was observed in 1(3%), respectively. Pretreatment TRAb and TW in a patient with persistent hyperthyroidism were 63.8(%) and 218(g),respectively. One female patient suffered from relapse of hyperthyroidism after her 2nd baby delivery, in the remaining 35 patients (89%), Hypothyroidism was achieved within 5 years after RAI. Average value of TRAb at 5 years and 9 years after RAI were 72%(38.0 %vs. 53.0 %, p-0.04) and 29%(15.3 vs.53.0, p=0.003), respectively. TRAb was still positive in 7 patients (hypothyroidism/hyperthyroidism/euthyroidism: 4/2/1) 9 years after RAI. Average value of TW at 5 years and 9 years after RAI were 12%(6.7g vs. 56.2g, p=0.013), and 9%(5.3g vs. 56,2g, p=0.009), respectively. TW is bigger than 20g in 2 patients (hypothyroidism/hyperthyroidism:1/1) 9 years after RAI. Worsening of TAO was not observed in any of patients.

Conclusions: After a single RAI, both TRAb and TW were significantly decreased compared with the pretreatment value during the 9-years follow up. Hypothyroidism was achieved in about 90% of the patients. Compared with TRAb, TW showed quicker decrease and reached about 9% of the pretreatment value. In contrast, TRAb remained positive 9years after RAI in more than 10% of the patients who attained euthyroidism or hypothyroidism.