thymic hyperplasia is a known feature of graves disease in adults ( 1 ) , but is rarely reported in children . murakami et al . 
 demonstrated an association between thymic size and graves disease by computed tomography 
 ( ct ) . their study in adults showed that thymic size and density were significantly increased 
 in untreated patients with graves disease and that the size and density of the thyroid 
 decreased markedly after 5 to 24 mo of treatment with antithyroid drugs ( 2 ) . therefore , thymic hyperplasia with graves disease 
 will regress in response to treatment with appropriate antithyroid drugs , and its 
 recognition would eliminate a thymus biopsy or thymectomy to establish the diagnosis ( 3 ) . this report presents the case of a 10-yr - old boy with thymic hyperplasia associated with 
 graves disease . the anterior mediastinal mass shrank markedly after 3 wk of treatment with 
 an antithyroid drug , thus allowing the patient to avoid invasive diagnostic procedures . a 10-yr - old boy with down syndrome was referred to the hospital for treatment of pneumonia . 
 he had a surgically repaired atrioventricular septal defect . poor weight gain ( 1.2 kg gain 
 from 7 to 10 yr old ) and increased sweating had been noted for about 3 yr . the family 
 history was negative for thyroid abnormalities . his height was 119.0 cm ( 3.15 sd ) , body 
 weight was 17.0 kg and body mass index was 12 kg / m . his body temperature was 
 37.5c , heart rate was 120 beats / min and respiratory rate was 30 breaths / min . each lobe was 
 approximately 3  2 cm in size , and a systolic bruit was heard over the thyroid gland . the serum thyroid stimulating hormone ( tsh ) level was less than 0.03 iu / ml , free 
 t3 level was 27.55 pg / ml and free t4 level was higher than 6.00 
 ng / dl . the thyroid - stimulating antibody level was elevated ( 195% ) , thus leading to the 
 diagnosis of graves disease . a chest ct scan 
 revealed an anterior mediastinal mass measuring 3  3 cm ( fig . c : after 6 mo of treatment . a ct scan three weeks after initiation of methyl - mercaptoimidazole ( mmi ) showed shrinkage 
 of the mass , thus supporting the diagnosis of thymic hyperplasia with graves disease ( fig . a follow - up ct scan obtained after 6 mo of treatment showed 
 continued shrinkage of the mass ( fig . although the precise mechanisms of thymic hyperplasia in graves disease are uncertain , an 
 immunological mechanism is suspected . several autoimmune diseases , including myasthenia 
 gravis , are also associated with thymic hyperplasia , suggesting a contribution of autoimmune 
 reactions . reported abnormally elevated serum 
 immunoglobulin levels in a patient with graves disease who had an enlarged thymus with 
 increased thymocyte proliferation in 1988 ( 4 ) . in 
 contrast , another report suggested that thymic hyperplasia is directly caused by elevated 
 thyroid hormone levels , because scheiff et al . showed that triiodothyronine 
 induced thymic hyperplasia in an animal model ( 5 ) . 
 according to other reports , it is likely that thymic hyperplasia develops secondary to 
 graves disease because thymectomy has no apparent effect on the course of graves disease 
 ( 7 , 8) . pubmed and igaku chuo zasshi ( in japanese ) list 45 japanese cases of thymic hyperplasia 
 associated with graves disease for between 1983 and 2007 . twenty - five of these 45 reported cases 
 had no other autoimmune diseases and are summarized in table 1 . one child case was not included because she was also complicated with 
 myasthenia gravis . the mean age of the 25 patients was 29 yr , ranging from 16 to 57 yr . the other 5 patients 
 underwent thymectomy because the mass did not shrink after treatment with antithyroid drugs . however , of the 5 patients , 3 still maintained a hyperthyroid status at thymectomy . in 
 addition , the follow - up period after the start of antithyroid therapy was relatively short 
 ( less than 3 mo ) in the other 2 patients . the mean duration before regression of the 
 mass was 4.7 mo ( range 39 mo ) . these cases suggest that a second radiological assessment to 
 confirm regression of the mass should be performed more than 3 mo after of antithyroid 
 therapy and with a normal thyroid status , while the current child case showed prompt 
 shrinkage of the mass . table 1 summary of 25 patients with thymic hyperplasia associated 
 with graves diseasenumber of patientssexmale 6female 19treatment thymectomy7biopsy + atd8atd10result immediate thymectomy2atd + thymectomy5regression 18observation period none2 before thymectomy 3 mo2 > 3 mo3*atd indicates antithyroid drug . * atd indicates antithyroid drug . in summary , thymic hyperplasia should be considered before invasive diagnostic procedures in patients that present with an anterior mediastinal mass associated with graves disease , even in children . further studies are therefore needed to fully understand the features of thymic hyperplasia with graves disease in children .
thymic hyperplasia associated with graves disease is rarely reported in children , 
 although it is not uncommon in adults . 
 occasionally , an enlarged thymus presents as an 
 anterior mediastinal mass on a radiographic examination . 
 such patients often undergo 
 invasive procedures such as a thymus biopsy or thymectomy because of suspected malignancy . 
 however , an enlarged thymus with graves disease is known to shrink after treatment with 
 antithyroid drugs 
 . therefore , recognition of this benign course would avoid unnecessary 
 surgical resection . 
 this report presents the case of a 10-yr - old boy with graves disease 
 complicated with an anterior mediastinal mass . 
 computed tomography showed a homogenous 
 mass with no invasion into the surrounding tissue . 
 a gallium-67 scintigraphy showed no 
 abnormal uptake . 
 shrinkage of the mass after treatment with an antithyroid drug 
 ( methyl - mercaptoimidazole ) supported the diagnosis of thymic hyperplasia with graves 
 disease . 
 this case report illustrates two important points . 
 first , pediatricians should be 
 aware that thymic hyperplasia can coexist with graves disease , even in children . 
 second , 
 close radiographic assessment would support a diagnosis of thymic hyperplasia and 
 eliminate invasive diagnostic procedures .