Subacute thyroiditis is a rare disease in childhood and is considered postviral 
in origin. Epstein-Barr virus (EBV) involvement has been suspected, but just 
once demonstrated in an adult female. We report a case of subacute thyroiditis 
during infectious mononucleosis in a 3-year-old girl. The diagnosis of 
infectious mononucleosis was supported, in addition to the typical clinical 
symptoms, by presence of EBV DNA both in plasma and leukocytes. Subacute 
thyroiditis was diagnosed based on elevated thyroid hormone levels with 
thyrotropin (TSH) suppression, high inflammation markers and almost absent 
(99)TC uptake by thyroid. Moreover after 3 months from diagnosis thyroid 
function went back to normal as well as the (99)TC uptake.