Objective: Subacute thyroiditis (SAT) is a transient inflammatory disorder of the thyroid gland, whereas Hashimoto thyroiditis is an autoimmune disorder. The objective of this report is to describe a patient with subacute thyroiditis with eventual evolution into Hashimoto thyroiditis. Methods: We report a case of a 50-year-old man with SAT diagnosed clinically and confirmed with technetium-99m thyroid scan, with eventual evolution into Hashimoto thyroiditis with increased anti–thyroid peroxidase antibodies. Results: A 50-year-old man with no prior comorbidities presented with high-grade fever with painful neck swelling for 10 days. On examination, patient was tachycardic, with fine tremors and a tender multinodular goiter but no bruit. Initial labs showed serum thyroid-stimulating hormone 0.084 μIU/mL (normal, 0.4 to 4.2 μIU/mL), free thyroxine 2.51 ng/dL (normal, 0.89 to 1.76 ng/dL), erythrocyte sedimentation rate 100 mm/hour, along with technetium-99m pertechnetate showing reduced tracer uptake (0.19%) on thyroid scan. Patient was diagnosed with SAT and was started on steroids. After 1 month, patient developed rapid fatigue and weight gain, for which thyroxine replacement was started. Despite 12 months since the diagnosis of SAT, patient continues to remain hypothyroid on thyroxine replacement, with development of a small, firm goiter and increased antibody titers, favoring an autoimmune disease process. Conclusion: The resolution of an inflammatory thyroid condition into an autoimmune disorder could be the result of homologies between thyroid antigens and bacteria and various susceptibility genes, contributing to this unusual phenomena; or, this could be the result of chance occurrence of two thyroid disorders.