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Thyroid peroxidase (TPO) is a key enzyme in the synthesis of thyroid hormone. TPO is involved in thyroid hormone synthesis (organification and coupling reactions). TPO is a major antigen corresponding to thyroid-microsomal autoantibodies. Anti-TPO auto antibodies are very important to diagnose autoimmune thyroid diseases and also in estimating its clinical course. Autoimmune thyroid disease is detected mostly by measuring circulating antibodies to thyroglobulin which is uncommon measurement of antibodies to TPO that gives reliable information about autoimmune thyroid disease. Eighty percent of Grave's disease patients have high levels of antiTPO antibodies. About 4% of subclinical hypothyroid patients with positive TPO antibodies develop clinical hypothyroidism. There is always a controversy on the relationship between breast cancer and thyroid disorders. As these tissues, i.e., breast and thyroid, originate embryologically from the same type of cells, hypothyroid/hyperthyroid females are more prone to develop benign or malignant breast tumors. The studies on breast cancer patients indicate increased thyroid disorders in breast cancer patients, most commonly Hashimoto's thyroiditis accounts to increased thyroid disorders in these patients. This is independent of hormonal receptor status of the patient. These findings suggest the usefulness of screening for thyroid disease in any patient with breast cancer.
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