TPO antibodies (anti-TPO): what a positive thyroid antibody result means

Autoimmune thyroid disease arises when the immune system mistakenly recognises parts of the thyroid as foreign. Lab Tests Online UK explains that the resulting autoantibodies can lead to chronic inflammation of the thyroid, tissue damage and disrupted function. Thyroid peroxidase is an enzyme in thyroid cells that helps make thyroid hormones, and antibodies against it, TPOAb, are the most reliable marker of autoimmune thyroid disease.

The thyroid, a butterfly-shaped gland on the windpipe, makes thyroxine (T4) and triiodothyronine (T3), which regulate how fast the body uses energy. A feedback loop through thyroid-stimulating hormone from the pituitary keeps hormone levels stable; antibodies can disturb this and cause too little or too much hormone.

It is requested when thyroid function tests suggest dysfunction, when there is an enlarged thyroid (goitre), when symptoms suggest thyroid disease, and in some people with related autoimmune diseases such as lupus, rheumatoid arthritis or pernicious anaemia, who have a higher risk of autoimmune thyroid disease. Lab Tests Online UK also says it may be requested during investigation of infertility, and before starting drugs such as lithium, amiodarone, interferon alpha or interleukin-2 that carry a risk of hypothyroidism when antibodies are present.

The NHS notes that in the UK the main cause of an underactive thyroid is Hashimoto disease, in which the immune system attacks the thyroid, that it is most common in women and usually starts between the ages of 30 and 50.

No preparation is needed; blood is taken from a vein in the arm. Tell the doctor about medicines because certain drugs interfere with thyroid tests. The result is reported as a concentration with a laboratory-specific cut-off, and different methods give different numbers, so compare only with the reference limit printed on your own report.

A positive TPOAb with abnormal thyroid hormones supports autoimmune thyroid disease. Lab Tests Online UK says mild to moderately elevated levels may be found in a variety of diseases and must be interpreted with symptoms and other thyroid tests, that slight increases occur rarely in thyroid cancer, and that significantly raised levels most often indicate Hashimoto thyroiditis, primary hypothyroidism or Graves disease; in general the higher the level, the more likely autoimmune thyroid disease is present.

A positive result with normal TSH and free T4 does not mean you have thyroid disease now, but it signals that you are more likely to develop an underactive thyroid in future, so periodic thyroid function checks are often advised. A negative result makes autoimmune thyroid disease less likely but does not rule it out entirely.

If a pregnant woman has a known autoimmune thyroid disease or an autoimmune condition linked to it, one or more thyroid antibodies may be requested, normally once at an early stage and again near the end. Thyroid antibodies from the mother can cross the placenta and increase the risk of hypothyroidism or hyperthyroidism in the foetus or newborn, so the results help doctors judge whether the baby may be at risk.

If hormones are abnormal, treatment follows the thyroid condition. For an underactive thyroid the NHS says treatment is usually levothyroxine tablets taken for life, with regular blood tests to check the dose. If hormones are normal, monitoring with TSH, often yearly, is common. See a GP for persistent tiredness, weight gain, feeling cold, low mood, hair loss or a swelling in the neck.