Autoantibodies to thyroperoxidase in blood

Reference values (norm) for the concentration of autoantibodies to thyroid peroxidase in blood serum are 0-18 IU/ml.

Thyroid peroxidase is an enzyme tightly bound to the granular endoplasmic reticulum of the epithelial cells of the thyroid follicles. It oxidizes iodides in the follicles to active iodine and iodizes tyrosine. During further oxidation by peroxidase, mono- and diiodotyrosines are conjugated to form various iodothyronines, of which tetraiodothyronine (T4) predominates quantitatively _. It has now been established that antibodies to antigens of the microsomal fraction are antibodies to thyroid peroxidase.

Determination of the concentration of autoantibodies to thyroid peroxidase is used as a marker of thyroid diseases caused by autoimmune processes. The concentration of antibodies in the blood is always elevated in Hashimoto's thyroiditis, Graves' disease and idiopathic myxedema.

In Hashimoto's thyroiditis, as a result of the destruction of thyroid peroxidase by autoantibodies in the thyroid follicles, iodine metabolism is disrupted, which leads to its low content in thyroglobulin. Thyroid function decreases mainly due to a decrease in T ₄ secretion.

When evaluating the obtained results of the study, it is necessary to take into account the so-called "cutoff" line, which is 18 IU/ml and is used to differentiate patients with a euthyroid state and patients with Hashimoto's thyroiditis and Graves' disease. In patients with Hashimoto's thyroiditis and Graves' disease, the content of antibodies to thyroid peroxidase of more than 18 IU/ml is detected in 98 and 83% of cases, respectively. The specificity of this limit for these diseases is 98%. Usually, the concentration of antibodies to thyroid peroxidase in the blood of patients with Hashimoto's thyroiditis and Graves' disease is 100 IU/ml and higher.

Since patients with autoimmune thyroiditis may have elevated levels of antibodies to thyroid peroxidase and/or thyroglobulin, it is advisable to determine them in combination to increase the reliability of laboratory diagnostics.

An increase in the concentration of antibodies to thyroid peroxidase in the blood can be detected in Riedel's thyroiditis and Addison's disease.

Clinical indications for testing antithyroid antibodies are as follows.

• Thyroglobulin antibodies.
  • Absolute indications: monitoring of postoperative treatment of thyroid cancer is mandatory in combination with thyroglobulin testing (to exclude false negative results); if the concentration of thyroglobulin in the blood serum is higher than 2.5-3 μg/l in patients who have undergone thyroid extirpation, it is necessary to exclude the presence of metastases and/or recurrence of cancer.
• antibodies to thyroid peroxidase.
  • Absolute indications: diagnosis of Graves' disease, autoimmune thyroiditis in primary hypothyroidism, prognosis of the risk of hypothyroidism with isolated increase in TSH, prognosis of postpartum thyroiditis in women from the high-risk group.
  • Relative indications: differential diagnostics of autoimmune (lymphocytic) and subacute thyroiditis in transient thyrotoxicosis, diagnostics of autoimmune thyroiditis in euthyroid diffuse or nodular goiter, prognosis of hypothyroidism in high-risk individuals. Repeated (during treatment) testing of antithyroid antibody levels in patients with established autoimmune thyroiditis is inappropriate, as they have no prognostic value. Patients with probable autoimmune thyroid disease in the absence of antibodies in the blood during the initial examination are shown to have them re-determined during the first and second years of observation.

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