A 44-year-old woman came to medical attention with right thyroid lobe enlargement and weight loss. Thyroid scan and ultrasound demonstrated a nodule in the right thyroid lobe; a right hemithyroidectomy was performed. Histologic examination documented granulomatous and lymphocytic thyroiditis associated with a follicular adenoma. No inflammation was apparent within the adenoma. The parallels between granulomatous thyroiditis and lymphocytic thyroiditis, including presumed viral initiation and deranged immunologic function, suggest that granulomatous thyroiditis may evolve into chronic lymphocytic thyroiditis; this case may be an example of such a transition. It is proposed that thyroid follicular adenomas have different antigenicity, which may account for the conspicuous absence of inflammation within the tumor.