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Background
Thyroglobulin (Tg), thyrotropin receptor, and thyroid microsomal antigen are the three forms of thyroid autoantigens that produce the antibodies in autoimmune thyroid diseases like Hashimoto thyroiditis or Grave’s disease.
Conditions which are associated with the TgAb like Grave’s disease, Hashimoto thyroiditis, myxedema, thyroid carcinoma, adenomatous hyperplasia, rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), chronic urticaria, autoimmune hemolytic anemia, type 1 diabetes, and Sjogren syndrome. 10 to 20 % of healthy people have serum antibodies against thyroglobulin antigens. Family history of having autoimmune thyroiditis and pregnant women can be positive for thyroglobulin antibody.
The thyroglobulin autoantigen that is high molecular weight glycoprotein contains iodine. It is released into the colloid of follicle cells within the thyroid gland. Thyroglobulin is a prohormone. It contains iodinated tyrosine moieties which combines with the tyrosine amino acid. This will produce the triiodothyronine (T3) and thyroxine (T4) active hormones. Thyroid hormones are crucial for the many metabolic processes in the body. The normal level of the thyroid hormone in healthy people is up to 30 ng/mL.
Thyroid follicles can produce thyroglobulin. Presence of thyroglobulin in a tissue biopsy sample provides the confirmation of the thyroid origin. Thyroglobulin test is important to determine the presence of recurrent, residual, or metastatic diseases in individuals who are previously diagnosis with a differentiated thyroid carcinoma.
Antithyroglobulin antibodies can be detected in about 20 % of the individuals and 10 % of the normal populations who have differentiated thyroid carcinoma. The result is not clinically dependable when thyroglobulin levels which are obtained when antithyroglobulin antibodies are present. Because of this reason, most of the laboratories do not publish the results of thyroglobulin when antithyroglobulin antibodies are present.
Indications/Applications
There are several clinical uses for thyroglobulin antibodies testing. It is performed if you have symptoms of thyroid diseases like weight gain, fatigue, dry skin, constipation, swelling, or muscle weakness.
To monitor the differentiated thyroid carcinoma
The serum thyroglobulin test is important to determine the presence or absence of recurrent, residual, or metastatic diseases in individuals who are previously diagnosis with a differentiated thyroid carcinoma mainly follicular or papillary thyroid carcinoma. Antithyroglobulin antibodies can be detected in about 20 % of the individuals who have differentiated thyroid carcinoma.
It is not known that how important thyroglobulin antibodies levels after the radioiodine ablation and surgery for differentiated thyroid carcinoma. The continuous presence of the thyroglobulin antibodies suggests that there may be a possibility of leftover thyroid tissue and a higher risk of recurrence. Other reason of continuous thyroglobulin antibodies is the correlation with the autoimmune thyroid disease.
Hashimoto thyroiditis
Autoantibodies attack the thyroid gland in Hashimoto thyroiditis. It is an autoimmune disease. Thyroglobulin antibodies are linked to this disease. Another antibody antimicrosomal antibody, also known as antithyroid peroxidase (anti-TPO), is highly specific for Hashimoto thyroiditis. The thyroid gland may be damaged as a result of antithyroglobulin. They are not specific in autoimmune thyroid diseases. Presence of thyroglobulin and absence of antimicrosomal antibodies is insufficient to detect the disease.
As per the study of 2030 patients who have antithyroglobulin and antimicrosomal antibodies is used to determine the Hashimoto thyroiditis. About 99% of the patients are positive for antimicrosomal antibodies. About 36 % of the patients are positive for antithyroglobulin antibodies. These results lead to the conclusion like both tests are costly and does not lead to any diagnosis benefit. So, thyroglobulin antibody test is not suggested to diagnose the Hashimoto thyroiditis.
Chronic urticaria
Thyroid autoantibodies are found in 10 to 33 % of the patients who have chronic urticaria. In general populations, it occurs 3 to 6 % of the time. Patients who have Hashimoto thyroiditis can experience urticaria more often.
Thyroid autoantibodies may cause the immune complex formation, complement activation, or start the release of proinflammatory substances in certain cases of urticaria.
Clinical Significance:
TgAbs bind to the thyroglobulin and affects the accuracy of thyroglobulin tests like radioimmunoassay (RIA) or immunometric assay. This will lead to the falsely low levels in immunometric assay. Low titer value of TgAb lead to the low level of thyroglobulin. RIA will lead to the flays high levels of thyroglobulin in the existence of TgAb.
This variation occurs because RIA detects both bound and unbound thyroglobulin while immunometric assay detects only free or unbound thyroglobulin. RIA may still identify the thyroglobulin, but accuracy may overestimate in the presence of TgAb. Some experts advised to use thyroglobulin RIA in the presence of TgAb because low level is more likely a normal and true level, but high level can overestimate the true Tg level.
Some laboratories use immunometric assay to detect the internet in the TgAb test. With this test, thyroglobulin is measured after and before the sample is raised with a known value of thyroglobulin. If the sufficient amount of thyroglobulin is found, then TgAb is not interfere and thyroglobulin level is reported. If the sufficient amount of thyroglobulin is not found, then it interferes with the test and thyroglobulin level is not reported.
As tgAb interference is important, it is necessary to include the TgAb measurement test in all thyroglobulin assays. Newly detected TgAb in patients who had differentiated thyroid carcinoma may previously have signal of recurrence of the cancer. In patients with differentiated thyroid carcinoma without autoimmune disorders, a persistence or elevation in the TgAb level may serve as a mimic tumor marker. As discussed earlier, this is because of RIA detects the bound and unbound thyroglobulin. So, the reported thyroglobulin level may be increased than the normal unbound circulating thyroglobulin level in serum.
Reference Range
Normally, antithyroglobulin is not found in the blood stream. 10 to 20 % of the healthy people have detectable level of antithyroglobulin.
The normal level of antithyroglobulin is below 116 IU/mL.
Interpretation
The conditions which are associated with the increased level of antithyroglobulin:
Differentiated thyroid carcinoma like Follicular Thyroid Carcinoma, Papillary Thyroid Carcinoma, and thyroid cancer
Chronic urticaria
Hashimoto thyroiditis
Conditioned which are associated with the antithyroglobulin:
Grave’s disease
Hashimoto thyroiditis
myxedema
adenomatous hyperplasia
rheumatoid arthritis (RA)
systemic lupus erythematosus (SLE)
chronic urticaria
autoimmune hemolytic anemia
type 1 diabetes
Sjogren syndrome
Collection And Panels
Sample type: Blood serum
Sample collection container: Red top tube
Sample collection method: Venipuncture
Sample volume: 1 mL
Panels: Antithyroglobulin and thyroglobulin are included in the thyroglobulin tumor marker panel. Other test along with this included are Thyroid-stimulating hormone (TSH) test, T4 test, and T3 test.
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