Thyroid echogenicity & radioiodine washout rates predicts outcome of radioidine therapy in Graves disease patients

Objectives To test the hypothesized that the response of the Graves disease (GD) to I-131 can be predicted by the disribution of I-131 within the gland and its retention time.

Methods We retrospectively audited reports of gland size,ultrasound (US) & thyroid uptake of all 534 GD(363 female)mean age 36.5(range 10 to 83)treated in our hospital in 7 years from Oct 2002.Pts were evaluated at 6 months after treatment & categorized as low dose responders (R1)or high dose responders (R2)if they responded to single dose up to 15 mCi or >15 mCi of I-131 respectively & resistant (RS)if they responded to more than single dose. Normal thyroid washout (NW) was defined as 4 hrs uptake < 24 hrs and rapid washout (RW) was defined as 4 hrs uptake equal or higher than 24 hrs uptake.US findings were categorized as normoechogenic(NE), hypoechoic(HO)or hyperechoic(HR).

Results RS glands were larger than responders with average RS,R1 & R2 sizes, 72, 53 and 50 mls respectively. 508 patients(95%) had thyroid uptake evaluated.421(83%)had NW consisted of R1 32%,R2 61%,RS 7% and 87(17%) pts had RW consisted R1(34%),R2(55%),RS(11%).The statistical analysis shows no significant difference.(P value for RS vs R1 is 0.5 & RS vs R2 is 0.253).Out of 111(21%) pts who had US, 66 were(NE),RS(8/12)67%,R1(13/26)50% & R2(45/73)62%,24(HO) patients,RS(2/12)17%,R1(10/26)38%,& R2(12/73)16% and 21(HR)pts RS(2/12)16%, R1(3/26)12% & R2(16/73)22%.Statistical analysis was significant. P value of 0.036.Out of 18 patients who combined (NE)and RW 73% were RS and 27% were both R1+R2.

Conclusions The (NE) thyroid gland have the most colloid content that may bind the I-131. Theoretically the combination of large gland, normoechogenicity with RW gives the least available I-131 for the active follicle cells resulting in maximum resistance to treatment., Hypoechoic and hyperechoic ultrasound features are associated with least colloid content and high radiosensitivity. The distribution of I-131 in the gland is more predictive of the treatment response than the I-131 washout rates alone