TPO antibodies and Hashimoto's thyroiditis

When the thyroid slows down, the cause is usually not something you did or ate. In most people it is the immune system. TPO antibodies are the blood test that shows this is happening.

What TPO antibodies are

TPO stands for thyroid peroxidase, an enzyme the thyroid uses to make its hormones. In some people the immune system mistakenly makes antibodies against it. These antibodies are a sign that the immune system is targeting the thyroid.

Over time, this immune attack can cause inflammation and gradual damage to the gland. As fewer working cells remain, the thyroid makes less hormone, TSH rises to push it harder, and eventually the thyroid may not keep up.

Hashimoto's thyroiditis

This autoimmune process is called Hashimoto's thyroiditis, or chronic autoimmune thyroiditis. Where diets contain enough iodine, it is the most common cause of an underactive thyroid. It is much more common in women than in men, often starts in middle age, and tends to run in families.

Hashimoto's usually develops slowly over years. Many people have no symptoms at first. Some notice a painless swelling in the front of the neck, called a goitre. When the thyroid does become underactive, the symptoms include:

• Tiredness and sluggishness.
• Feeling cold when others are comfortable.
• Weight gain without a change in eating.
• Dry skin, hair thinning and brittle nails.
• Constipation.
• Low mood and slowed thinking.
• Heavier or irregular periods.

A positive result without thyroid disease

TPO antibodies are surprisingly common. A sizeable minority of adults have them, more often women and older people, and many of them have normal thyroid function and never develop a problem.

When the antibodies matter most

TPO antibodies are most useful when they are read alongside TSH and free T4.

• With a raised TSH, positive antibodies make it more likely that the thyroid will continue to slow down, which can influence whether and when a doctor suggests treatment.
• With a normal TSH, positive antibodies are a reason for periodic monitoring rather than treatment.
• Before or during pregnancy, positive antibodies matter more, because pregnancy increases the demand on the thyroid. If you are pregnant or planning a pregnancy and your antibodies are positive, tell your doctor.
• With symptoms of an overactive thyroid, TPO antibodies can also be raised in Graves' disease, although a different antibody test is used to confirm that diagnosis.

Reading the number

TPO antibody results are reported in IU/mL, and the cut-off for a positive result varies between labs and methods. LifePrint uses the range printed on your lab report. A higher level suggests a more active immune process, but the exact number matters less than whether it is positive and what your TSH is doing.

Unlike TSH, TPO antibodies do not usually need to be measured again and again. Once they are known to be positive, TSH and free T4 are the tests used to follow the thyroid over time. The antibody level does not guide treatment.

Links with other autoimmune conditions

Autoimmune conditions tend to cluster. People with Hashimoto's are somewhat more likely to have others, such as type 1 diabetes, coeliac disease, vitiligo or pernicious anaemia, the autoimmune cause of low B12. If you have positive TPO antibodies and symptoms that suggest one of these, mention it to your doctor.

Can you prevent or reverse it?

There is no proven way to switch off the antibodies or stop Hashimoto's progressing. Diets and supplements promoted for this purpose have limited evidence, and very high iodine intake can make things worse. When the thyroid does become underactive, treatment with replacement hormone is well established and effective, and a doctor will guide it.

What happens next

Your LifePrint report explains your TPO antibody result together with your TSH, free T4 and free T3, and flags how urgently to follow up. If your antibodies are positive, share the result with a registered healthcare practitioner so they can decide how often your thyroid should be checked.