Autoimmune thyroiditis can be confirmed in a patient by

Once hypothyroidism is diagnosed by clinical features, TSH and free T4 measurements, etiology can be confirmed by measuring the presence of autoantibody—particularly thyroid peroxidase (TPO), which is present in 90 to 95% of patients with autoimmune hypothyroidism. Biopsy by fine needle aspirate can confirm the diagnosis but is not necessary in most cases.