TPOAb is more sensitive than AMA and thyroglobulin
In the second study comparing AMA to TPOAb, the thyroid antibody test results of 32 healthy patients were compared with those of 262 clinic patients. In those with known thyroid dysfunction, TPOAb was found to be a more sensitive assay than AMA for autoimmune thyroid disorders. The sensitivity of TPOAb levels >3.1 U/mL was 88.1%; AMA sensitivity was 70.2% (P<.001).2,3
A cross-sectional study (National Health and Nutrition Examination Survey [NHANES III]) evaluated the presence of thyroid antibodies in 17,353 people representing the geographic and ethnic distribution of the United States, 95% of whom were categorized as free of thyroid disease.4 The study found that TPOAb was more sensitive than thyroglobulin for diagnosing nonspecific thyroid disease. The diagnosis of thyroid disease was based on abnormal TSH and free T4 levels. Abnormally high levels of TPOAb had an LR+ of 4.3 and LR– of 0.6 (P<.0001) for thyroid disease, compared with an LR+ of 3.4 and LR– of 0.7 (P<.01) for abnormally elevated thyroglobulin.
TSH + TPOAb more accurate than TSH in women
In the early 1970s, a cohort study of 2779 adults from Great Britain attempted to establish the incidence of thyroid disease in the general population by measuring TSH and TPOAb. Twenty years later, investigators restudied 1708 people from the original sample to determine the incidence of hypothyroidism and the prognostic value of these 2 biochemical markers for its development. At follow-up, the definition of a new case of hypothyroidism was based on an “intention to treat by the general practitioner by meeting clear biochemical criteria and/or symptoms.”
The initial presence of abnormally high serum TPOAb levels and TSH >2.0 mU/L predicted a 4.3% annual risk of developing hypothyroidism compared with a 2.6% annual risk with serum TSH >6.0 mU/L alone in women. This risk was not estimated for men because of the small number of cases.5
Recommendations
The American Association of Clinical Endocrinologists (AACE) makes no specific recommendations about laboratory testing of thyroid antibodies. Based on clinical judgment, the AACE states that antibodies may be considered in the workup of hyperthyroidism and hypothyroidism and to determine potential risk to the fetus in pregnant women diagnosed with Graves’ disease.6
The National Academy of Clinical Biochemistry (NACB) recommends TPOAb measurements in patients who have Down syndrome, are pregnant, or have miscarried or failed in vitro fertilization. The NACB also advocates measuring TPOAb before treatment with amiodarone, lithium, interferon-α, or interleukin-2.7