Over a one month period, a 35 year old man develops symptoms of hypehyroidism with tachycardia and heat intolerance. Thyroid biopsy demonstrates a heavy mononuclear cell infiltrate with multinucleated giant cells, follicular disruption, and loss of colloid. About one month later,the patient develops symptoms of hypothyroidism. To which of the following is this patient's condition thought to be most closely related?