Thyroid
Anti-Tg
Antibodies against thyroglobulin, a protein from the thyroid; found in autoimmune diseases of the thyroid.
Everything about this testWhat it measures
Thyroglobulin (Tg) is a protein that is made only in the thyroid and in which thyroid hormone is built. In an autoimmune reaction against the thyroid, the immune system can make antibodies against this protein: anti-Tg. The test measures how many of these antibodies are in the blood. In the hospital, anti-Tg is mainly measured alongside thyroglobulin, for monitoring after removal of the thyroid, because the antibodies can interfere with that measurement.
- A sign of an autoimmune disease of the thyroid, such as Hashimoto's disease
- Interference with the thyroglobulin measurement by antibodies (hospital setting)
High
A raised anti-Tg occurs in various thyroid diseases, especially in autoimmune diseases such as Hashimoto's disease and Graves' disease. People without thyroid disease can also have raised anti-Tg, without this having consequences. A raised value says nothing about how well the thyroid is working now; TSH and free T4 show that. With normal thyroid function, a raised anti-Tg on its own usually means nothing for treatment.
Low
Most people have no anti-Tg; a low or undetectable value is normal. This does not rule out an autoimmune disease of the thyroid: in Hashimoto's disease, anti-TPO is found more often than anti-Tg.
Preparation
- Do you take biotin (vitamin B8, often in hair and nail supplements)? Stop at least 2 days before the sample is taken, or 4 days for a high dose.
If your result is abnormal
With a raised anti-Tg, the GP looks at TSH and free T4. If these are normal, there is usually no further testing; with symptoms that point to a thyroid disorder, TSH can be measured again later.
Reliability
Anti-Tg says nothing about how well the thyroid is working; thyroid function is determined with TSH and free T4, according to the Dutch GP guideline (NHG) on thyroid disorders. According to the Dutch Society for Clinical Chemistry (NVKC), the test is mainly intended to assess whether a thyroglobulin measurement in the hospital is reliable. As a sign of an autoimmune disease of the thyroid, anti-Tg adds little alongside anti-TPO.
Limitations
- People without thyroid disease can also have raised anti-Tg.
- Reference values and cut-off values differ per laboratory and measurement method; only compare results from the same laboratory.
- The result says nothing about current thyroid function; without TSH alongside it, it is hard to interpret.
How common is this
An underactive thyroid affects about 3 to 4 in 100 adults in the Netherlands and an overactive thyroid about 1 in 100, both more often women. How often anti-Tg occurs in people without symptoms is not well known.
Who is this useful for
The result is most meaningful in the hospital, alongside a thyroglobulin measurement after removal of the thyroid. With thyroid symptoms, TSH shows how the thyroid is working; anti-Tg says nothing about that. As a sign of an autoimmune disease of the thyroid, TSH and anti-TPO usually say more than anti-Tg.
Downsides and risks
- A raised anti-Tg with normal thyroid function can lead to unnecessary worry or unnecessary follow-up testing.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results