TPO antibodies (anti-TPO)

TPO antibodies (anti-TPO, formerly microsomal antibodies) are autoantibodies: proteins of the body's own immune system that are directed against thyroid peroxidase, an enzyme of the thyroid. They are measured in serum, usually together with the thyroid values TSH, FT3 and FT4. In the autoantibody screens for long/post-COVID and chronic fatigue syndrome they are part of a larger group of autoantibodies. The value does not show how much thyroid hormone is made, but whether the immune system is turning against the thyroid – which can precede changes in hormone production by years.

What thyroid peroxidase does in the body

Thyroid peroxidase sits on the surface of the thyroid cells that faces the inside of the follicles. There it activates the iodine taken up from the blood and incorporates it into the amino acid tyrosine of the storage protein thyroglobulin. It then links two of these iodinated building blocks at a time to form the hormones thyroxine (T4) and triiodothyronine (T3). Without the enzyme, the thyroid cannot make hormones. Because thyroid peroxidase occurs only in the thyroid, antibodies against it are a fairly specific sign that a defence reaction is directed against this organ.

Low TPO antibodies – what does it mean?

A low or undetectable value is the normal case. It speaks against the immune system forming antibodies against the thyroid to any significant extent. An autoimmune thyroid inflammation is not entirely ruled out, however: in a small proportion of those affected no TPO antibodies can be detected, but sometimes antibodies against thyroglobulin are found, or the changes show only on ultrasound. The value is therefore always considered together with TSH, the free thyroid hormones and, where relevant, the thyroglobulin antibodies. A value that is once low usually remains stable, but can change in phases of life with hormonal changes.

Raised TPO antibodies – what happens in the thyroid?

Raised TPO antibodies show that immune cells have migrated into the thyroid. The higher the value, the more pronounced this migration of lymphocytes usually is. The actual damage comes mainly from T cells; the antibodies can additionally activate the complement system. Over years, thyroid cells are lost in this way, hormone production declines and TSH rises – an underactive thyroid develops, with tiredness, sensitivity to cold, weight gain and dry skin. This is typical of Hashimoto's thyroiditis; the antibodies are also often found in Graves' disease and after childbirth. Raised values occur more often in women than in men and increase with age, even while function is still normal.

The interpretation is given in your report.