TPO antibodies

Your thyroid makes hormone by building iodine into a protein, and the enzyme that does that is called thyroid peroxidase, TPO for short. In autoimmune thyroid disease your immune system mistakes that enzyme for an intruder and makes antibodies against it. Those antibodies are not the attack itself; they are the trace of an immune system that has entered the thyroid. The attack itself, by immune cells that slowly break down the thyroid tissue, is what makes the thyroid underactive over years. That is Hashimoto's disease, the most common reason for an underactive thyroid. That is why TPO antibodies say something different from TSH and free T4. Those two show how your thyroid is doing right now; the antibodies show whether an autoimmune process is under way. With a normal thyroid function, antibodies mean a raised chance that the thyroid becomes underactive in the years after, roughly a few percent a year, and with a slightly raised TSH they make it more likely that it progresses. With a clearly underactive thyroid they confirm the cause, but the treatment is the same, with or without antibodies. Your report shows TPO antibodies in kU/L, and the limit differs per lab because the tests are not on one scale: from 20 kU/L at the strictest lab to 35 kU/L at the widest. Below the limit on your own report no antibodies were detectable, and that is the healthy outcome.

TPO antibodies answer the question why, and that is a question that changes something less often than you would think. With an underactive thyroid with a high TSH and a low free T4 the treatment is thyroid hormone, whether the antibodies are detectable or not; that is the reason the test is usually skipped in general practice. Where they do add something is with doubt: a slightly raised TSH with a normal free T4 usually returns to normal by itself, and antibodies make the chance that it progresses a good deal larger. That difference decides whether you wait or treat earlier. There is a second situation in which the antibodies count: a wish for a child or a pregnancy. Women with TPO antibodies and a normal thyroid function have miscarriages somewhat more often and more often a thyroid inflammation in the year after the birth, and their thyroid has less reserve in pregnancy. That is a reason to follow TSH in pregnancy, not to treat the antibodies. And there is a pitfall. One in ten healthy adults, especially women and older people, has detectable TPO antibodies without ever getting thyroid symptoms. Antibodies are a risk, not a diagnosis, and a raised value with a normal TSH is a reason to have TSH measured once a year, not to feel ill.

A low value does not exist here as a finding. Below the limit on your own report no antibodies were detectable, and that is the healthy outcome. A negative result does not completely rule out Hashimoto's, by the way, because a small share of people with an underactive thyroid from autoimmune disease has no detectable TPO antibodies; then TSH and free T4 say enough. A raised value means your immune system is directed at the thyroid, and TSH and free T4 say what that means right now. With a high TSH and a low free T4 it is Hashimoto's, and the treatment is thyroid hormone. With a slightly raised TSH and a normal free T4 it is a thyroid that still keeps up but will probably become underactive, and then you follow TSH every six months to a year. 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 runs first overactive and then underactive and usually recovers by itself. How high the value is says little: above the limit the presence counts, not the number, and the numbers of different labs cannot be compared. Set TPO antibodies beside TSH and free T4, in that order: first how the thyroid is doing, then why. Tg antibodies are the second thyroid antibody, which arises in the same process and rarely adds anything.