A 38-year-old mother of three presents to her primary care office with complaints of fatigue. She feels that her energy level has been low for the past 3 months. She was previously healthy and taking no medications. She does report that she has gained about 10 lb and has severe constipation, for which she has been taking a number of laxatives. A TSH is elevated at 25 mU/L. Free T4 is low. She is wondering why she has hypothyroidism. Which of the following tests is most likely to diagnose the etiology? A. Antithyroid peroxidase antibody
B. Antithyroglobulin antibody
C. Radioiodine uptake scan
D. Serum thyroglobulin level
E. Thyroid ultrasound

A. Antithyroid peroxidase antibody

Explanation:

The most common cause of hypothyroidism is autoimmune thyroiditis, also known as chronic lymphocytic thyroiditis or Hashimoto's disease. This condition (autoimmune thyroiditis) is caused by damage of the thyroid gland, which occurs when antibodies of the immune system attack this gland. The thyroid peroxidase test is an enzyme immunoassay widely used to measure the level of autoantibodies against thyroid peroxidase. Patients with chronic thyroiditis often display a positive thyroid peroxidase test. On the other hand, thyroid ultrasound is useful for the initial evaluation of painless thyroiditis in order to distinguish nodules or multinodular goiter. Antithyroid peroxidase antibodies are commonly observed in individuals suffering autoimmune thyroiditis, whereas antithyroglobulin antibodies are observed in different thyroid disorders and autoimmune diseases. Finally, a radioiodine uptake scan measures the amount of radioactive iodine accumulated in the thyroid gland, which is useful only for diagnosing early‐stage of Hashimoto's thyroiditis.