A 34-year-old woman was referred to our hospital complaining of sore throat and 
arthralgia. She had low-grade fever, tachycardia, and goiter with tenderness. 
Laboratory data revealed thyrotoxicosis and tests for acute inflammatory markers 
were positive. Thyroidal radioactive iodine uptake was below normal. 
Ultrasonography of thyroid revealed mild thyroid enlargement and hypoechogenic 
areas consistent with tenderness. Subacute thyroiditis was diagnosed and 
prednisone was administered. Two years later, her identical twin sister, who 
lives separately, was referred to our hospital because of neck pain, low-grade 
fever, and palpitation. She exhibited the same clinical picture as her twin 
sister, and was also diagnosed as having subacute thyroiditis. Although the 
cause of subacute thyroiditis remains unclear, viral infection has been 
implicated in the onset of subacute thyroiditis in genetically predisposed 
individuals. We could not identify the viruses, but heterozygotes for HLA-B35, 
which has been reported to be linked with subacute thyroiditis, were found in 
the twins. This supports the suspicion that genetic factors, including this HLA 
haplotype, play a critical role in the onset of subacute thyroiditis.