What is the best test for diagnosing Hashimoto's thyroiditis?

Hashimoto's thyroiditis is an autoimmune condition that specifically leads to the destruction of the thyroid gland. The most accurate test for diagnosing this disorder is the measurement of antithyroid peroxidase (anti-TPO) antibodies, also referred to as antimicrosomal antibodies. These antibodies are produced by the immune system in response to the thyroid tissue being attacked. Elevated levels of anti-TPO antibodies are commonly found in patients with Hashimoto's thyroiditis and serve as a hallmark for this diagnosis.

While free T4 levels and thyroid-stimulating hormone (TSH) tests are important for assessing thyroid function, they do not specifically diagnose Hashimoto's thyroiditis. Free T4 levels evaluate the amount of active thyroid hormone in the blood, and TSH measures how much thyroid hormone the pituitary gland is signaling the thyroid to produce. Both tests help in understanding thyroid status but do not provide information about the autoimmune nature of the disease.

A radionuclide thyroid scan assesses thyroid function and morphology but is not specific for Hashimoto's thyroiditis. In this condition, the scan would typically show a reduced uptake of the radioactive iodine due to decreased thyroid function, but it would not confirm the diagnosis.

The measurement of anti-TPO antibody levels is the