Higher magnification of the previous case showing numerous foreign body giant cells and chronic inflammatory infiltrate consisting of lymphocytes and plasma cells. A few residual thyroid follicles with colloid can be seen. The acute phase of the disease may be associated with mild hyperthyroidism which is usually followed by a period of hypothyroidism. An association with HLA-B35 haplotype has been found. Etiology is unknown but a viral infection may be the trigger. Most cases resolve completely and require only symptomatic treatment.