In the not so distant past, elevated TSH and Antithyroid microsomal antibodies (TMAb) were considered the gold standard for the diagnosis of chronic autoimmune (Hashimoto's) thyroiditis. despite the presence of contaminating antigens in the preparations. Thyroid peroxidase (TPO), a large, membrane-associated glycoprotein expressed only in the thyrocytes, is an important enzyme involved in the multistep thyroid hormone synthesis. It is the major autoantigen in the thyroid microsomal particle. Assays for Thyroid peroxidase antibody (TPOAb) are now available and this has rendered TMAb obsolete.
Antithyroid microsomal antibodies in titers greater than 1:6,400 are strongly suggestive of chronic autoimmune (Hashimoto) thyroiditis or Graves disease (GD). TPOAb in excess of 200 IU per mL, strongly suggests Hashimoto's thyroiditis. 57-74% of patients with Graves' disease also show high values.
TMAb titers >1:100 are found in 10-13% of elderly women (mean age 80 years)and 3% of elderly men, but titers >1:6400 are present in only 1% of normal individuals.
Detection of TPOAb is strong evidence against a goiter being due to non-autoimmune causes of hypothyroidism. The risk for the development of hypothyroidism is 3% to 4% per year if TPOAb are present and TSH is elevated.
By the way, do you have a goiter? You should get your TPOAb levels checked, to be sure.
MB BS, MS, FRCS(Edinburgh)