Thyroglobulin antibodies (anti-TG) test

Anti-TG are antibodies that the immune system mistakenly directs against thyroglobulin — a protein that the thyroid cells use to produce thyroid hormones. When the body starts producing these antibodies, it points to an autoimmune process in the thyroid: the immune system recognises thyroid tissue as foreign and attacks it. Anti-TG is most often tested together with antibodies against thyroid peroxidase (anti-TPO), because both point to similar autoimmune processes, although anti-TPO is usually the more sensitive one. According to MedlinePlus, a positive anti-TG result can be linked with Hashimoto's thyroiditis, Graves' disease, subacute thyroiditis, reduced thyroid function, systemic lupus erythematosus or type 1 diabetes. The test is also used to monitor whether the disease has come back after treatment for thyroid cancer, because the presence of anti-TG can interfere with measuring thyroglobulin accurately — the marker of cancer recurrence. A negative result is considered normal: it shows that no antibodies against thyroglobulin were found in the blood.

• When a general thyroid hormone test (TSH, free T4) shows that the thyroid is not working properly, and the aim is to establish whether the cause is an autoimmune disease
• When Hashimoto's thyroiditis is suspected — with weight gain, tiredness, hair loss, intolerance of cold, constipation, low mood, joint pain
• When Graves' disease is suspected — with weight loss, bulging eyes, trembling hands, intolerance of heat, insomnia, anxiety, a faster heartbeat, goitre
• To follow the results of thyroid cancer treatment — whether the disease has come back after surgery or treatment
• When there are cases of autoimmune thyroiditis in the family, or the test is done during pregnancy because of the risk to thyroid function

A positive (raised) anti-TG result shows that the immune system is producing antibodies against the thyroid protein thyroglobulin. According to MedlinePlus, this is most often linked with autoimmune thyroid diseases — Hashimoto's thyroiditis or Graves' disease — but it can also occur in other autoimmune diseases, or in a healthy person whose close relative has autoimmune thyroiditis.

What can lie behind a positive result

• Hashimoto's thyroiditis (chronic lymphocytic thyroiditis) — the most common cause
• Graves' disease
• Subacute thyroiditis
• Reduced thyroid function (hypothyroidism)
• Systemic lupus erythematosus
• Type 1 diabetes
• Pregnancy
• Close relatives with autoimmune thyroiditis

Symptoms that can accompany a positive result

• Weight gain, tiredness, hair loss, intolerance of cold, irregular periods, constipation, low mood, joint pain (Hashimoto's thyroiditis)
• Weight loss, bulging eyes, trembling hands, intolerance of heat, insomnia, anxiety, a faster heartbeat, goitre (Graves' disease)

What to discuss with a doctor

• Assessing TSH, free T4 and anti-TPO antibodies at the same time — they help establish which autoimmune condition is likely
• Discussing symptoms and the family history of autoimmune diseases with a doctor
• If the test is done as part of monitoring thyroid cancer treatment — discussing with a doctor how the presence of anti-TG can affect the interpretation of thyroglobulin results
• Discussing with a doctor when to repeat the test, to follow how the measure changes over time

How anti-TG differs from anti-TPO

Both antibodies point to an autoimmune process in the thyroid, but they are directed against different proteins — thyroglobulin (anti-TG) and thyroid peroxidase (anti-TPO). According to MedlinePlus, anti-TPO is usually the more sensitive marker of autoimmune thyroiditis, which is why the two tests are often done together.

Why anti-TG is tested in thyroid cancer

According to MedlinePlus, anti-TG is used to monitor whether the disease has come back after treatment for thyroid cancer. The presence of these antibodies in the blood can interfere with measuring thyroglobulin accurately, and thyroglobulin is used as a marker of cancer recurrence, so the two measures are assessed together.