Clinical background:
Thyroid peroxidase (TPO) is a membrane bound glycosylated haem-containing protein found in the apical membrane of the thyroid follicular cells. TPO, the major component of a large protein known as thyroid microsomal antigen, catalyses the iodination of the tyrosyl groups in thyroglobulin resulting in the synthesis of thyroid hormones, T3 and T4. Autoimmune thyroid disease is characterised by the presence of autoantibodies against TPO. Anti-TPO antibodies activate complement and are thought to be significantly involved in thyroid dysfunction and the pathogenesis of hypothyroidism.
The measurement of autoantibodies against thyroid peroxidise is useful in identifying patients with autoimmune thyroid disease. Levels of anti-TPO antibodies are increased in greater than 90% of patients with active autoimmune thyroiditis. In patients with autoimmune thyroid disease, anti-TPO antibodies are present in nearly all patients with Hashimoto’s thyroiditis and in greater than 70% of patients with Grave’s disease. Anti-TPO antibodies are also present in patients with atrophic thyroiditis and primary myxoedema.
Specimen container paediatric:
Serum (SST or plain tube)
Specimen container adult:
Serum (SST or plain tube)
Minimum volume paediatric:
0.5 mL blood
Minimum volume adult:
1 mL blood
Sample stability:
Unseparated sample: unknown
Separated sample: unknown at 15-25C, 3 days at 2-8C, 1 month at -20C
Transport requirements:
Ambient
Interpretation:
Slightly elevated anti-TPO antibodies can be detected in about 10% healthy individuals with normal thyroid function: the clinical significance of this has not been determined. The measurement of anti-TPO is therefore of little clinical value in the presence of normal thyroid function tests, except in the investigation of patients with other autoimmune diseases in whom it may be important to detect thyroid involvement. In patients with borderline TSH values but normal free T4 levels, indicating subclinical hypothyroidism, raised anti-TPO antibodies is associated with an increased risk of progression to overt hypothyroidism. Anti-TPO antibodies are increased in women with postpartum thyroiditis, this condition occurring in 5 – 9% of postpartum women. Although postpartum thyroiditis is associated with anti-TPO antibodies, 50% of anti-TPO positive women do not develop thyroid dysfunction.
Reference ranges:
<34 kU/L
Other info:
Lithium heparin plasma samples also acceptable