The etiology of subacute (de Quervain's) thyroiditis (SAT) is uncertain, 
although it probably represents a nonspecific inflammatory response by the 
thyroid to a variety of viruses. It has been suggested that nonimmune processes 
are involved in SAT patients who have negative autoantibody titers. The disease 
has a variable course; although it is self-limited in most cases, some patients 
develop transient hypothyroidism, and others do not during the recovery period. 
The present study was performed to evaluate the occurrence of TSH receptor 
antibody (TRAb), measured by RRA (TSH binding inhibitor), TRAb measured by 
stimulation assay (thyroid-stimulating antibody), and TRAb measured by blocking 
assay [TSH-blocking antibody (TSH-BAb)] activity in 68 patients with SAT who had 
negative autoantibody titers. The patients were divided into 2 groups: group I, 
31 patients who developed hypothyroidism during the recovery period; and group 
II, 37 patients who remained euthyroid during recovery. Positive immunoglobulin 
activity occurred in about 20% of group I patients during follow-up, but in only 
3% of group II patients. About 20% of group I patients developed positive 
TSH-BAb activity and were hypothyroid, requiring exogenous hormone therapy for 
1.2-3.5 yr, whereas hypothyroidism was relatively transient in group I patients 
who had negative TSH-BAb activity (2-6 months). Although increased TSH-BAb 
activity may account for hypothyroidism in some patients with SAT, the precise 
mechanism for the development of transient hypothyroidism in SAT remains 
enigmatic.