The patient with symptoms including tachycardia and thyroid enlargement has T3 and T4 elevated with a suppressed TSH. Which antibody is most likely present?

• A. Hashimoto disease antibodies
• B. Addison disease antibodies
• C. Circulating antibodies to thyroid peroxidase
• D. Pituitary gland antibodies

Answer: (C)

When the thyroid is overactive (high T3/T4 with suppressed TSH) and the gland is enlarged, this points to autoimmune thyroid disease as the underlying cause. Antibodies to thyroid peroxidase are a common autoimmune marker in thyroiditis and can be present in autoimmune thyroid conditions that cause hyperthyroidism as well as those that cause hypothyroidism. Even though the main pathogenic antibody in Graves disease is the TSH receptor–stimulating antibody, anti-thyroid peroxidase antibodies often coexist in autoimmune thyroid disorders, making them the most likely antibody detected among the options provided.

Hashimoto antibodies are more typically linked to Hashimoto thyroiditis and hypothyroidism, not this hyperthyroid picture. Addison disease antibodies pertain to adrenal autoimmune disease, and pituitary gland antibodies would affect pituitary function rather than the thyroid directly. Therefore, circulating antibodies to thyroid peroxidase best fit an autoimmune thyroid process presenting with hyperthyroidism in this scenario.