Tg antibodies

Your thyroid makes hormone by building iodine into a large protein, thyroglobulin, and stores that protein in small vesicles until it is needed. It is the thyroid's storage cupboard. In autoimmune thyroid disease your immune system is directed at the thyroid, and makes antibodies against what it encounters there: against the enzyme TPO and against the storage protein thyroglobulin. Tg antibodies are the second trace of the same process, and they almost always occur together with TPO antibodies. That is why they rarely add anything. TPO antibodies are more sensitive, detectable in more people with Hashimoto's and more usable as a measure of the chance that a thyroid becomes underactive; one in fifty people with Hashimoto's has only Tg antibodies, and for them they are the only trace. Where Tg antibodies have a role of their own is after treatment for thyroid cancer: thyroglobulin in the blood is then the measure of remaining thyroid tissue, and antibodies against it disturb that measurement, so they are always measured alongside. Your report shows Tg antibodies in IU/mL, and the limit differs per lab more than for any other value: from 10 IU/mL at the strictest labs to 115 IU/mL at the widest, because the tests are not on one scale. Below the limit on your own report the result is called negative, and that is the healthy outcome.

Tg antibodies are the value you usually do not need, and honestly that is the most important message of this page. Anyone who wants to know whether an underactive thyroid comes from autoimmune disease has enough with TSH, free T4 and TPO antibodies. Anyone who finds a positive Tg antibody without symptoms has the same as with TPO antibodies: a raised chance that the thyroid becomes underactive later, and a reason to measure TSH once a year, nothing more. There are two situations in which they do count. The first is the small group with Hashimoto's in whom TPO antibodies are not detectable: with a high TSH without TPO antibodies, Tg antibodies sometimes still show that it is an autoimmune process. The second is the follow-up after thyroid cancer, where thyroglobulin in the blood shows whether thyroid tissue has remained or returned, and where Tg antibodies can disturb that measurement; there they belong with every check, and there the trend of the antibodies themselves counts too. And there is the pitfall of the number. Because the limits differ more than tenfold, the same result can be called negative at one lab and positive at another, and the height above the limit says little. Anyone who wants to follow their antibodies has them measured at the same lab each time.

A low value does not exist here as a finding. Below the limit on your own report the result is called negative: no antibodies were detectable, and that is the healthy outcome. A negative result says nothing about your thyroid function itself; TSH and free T4 are for that. A raised value means your immune system is directed at the thyroid, and that is the same process as with TPO antibodies. With a high TSH and a low free T4 it is Hashimoto's, and the treatment is thyroid hormone, with or without antibodies. With a normal TSH it is a raised chance of later, and then you measure TSH once a year. In the year after a birth, antibodies fit a thyroid inflammation that usually recovers by itself. After treatment for thyroid cancer, Tg antibodies mean the thyroglobulin measurement cannot be trusted, and then the doctor follows the antibodies themselves: if they fall over the years, that is favourable, if they rise, that is a reason for further investigation. How high the value is says little outside that situation, and a result from one lab cannot be compared with another lab's limit. Read Tg antibodies as a pair with TPO antibodies, and both only after TSH and free T4: those two say how your thyroid is doing right now, and only then does an antibody add something about the why.