What Positive Thyroid Peroxidase Antibodies With a Normal TSH Mean

Antibodies are proteins produced by your body that respond to or attempt to destroy antigens, which are substances, such as viruses and bacteria, that the body identifies as foreign.

In some cases, your body mistakenly identifies your own glands, tissues, and organs as foreign. This type of reaction is characteristic of autoimmune diseases, and these antibodies, which launch a misguided attack on a person's own body, are called autoimmune antibodies.

One class of autoimmune antibodies are thyroid peroxidase (TPO) antibodies, and their presence ("positivity") suggests the presence of an autoimmune thyroid disease, most commonly Hashimoto's thyroiditis. 

In Hashimoto’s thyroiditis, the TPO antibodies attack your thyroid gland.

Over time, the TPO antibodies cause inflammation and eventually can destroy all or part of your thyroid gland. As the thyroid becomes less able to produce thyroid hormone, you may gradually become hypothyroid. Hashimoto’s antibodies can also cause your thyroid to form nodules, or become enlarged, known as a goiter.

Even so, it can take time for the destructive effect on your thyroid to be reflected in your thyroid-stimulating hormone (TSH) test level. It's not uncommon, in fact, to have positive TPO antibodies for months or years before your TSH level rises to a point where you are diagnosed with hypothyroidism.

It's important to mention, as well, that some people never progress to being hypothyroid, despite having positive TPO antibodies. 

Generally speaking, if your TSH test result is normal (between 0.4 and 4.5 milli-international units per liter, or mU/L), treatment (with a thyroid hormone replacement drug) is not warranted, even if TPO antibodies are positive.

However, if your TSH is mildly elevated (let's say hovering around 6.0mU/L) and your thyroxine (T4) hormone is normal, the "positivity" of TPO antibodies may sway your doctor into initiating thyroid hormone treatment. This is because early therapy may prevent the progression of this so-called subclinical hypothyroidism into overt hypothyroidism.

Your doctor may even be more swayed to treat your subclinical hypothyroidism if you have symptoms like fatigue, constipation, or depression, or you have another autoimmune disease, for example, celiac disease.

Age, as well, will play a role in your doctor's decision. Typically, there is a higher threshold for initiating thyroid hormone replacement medication in older adults, because their baseline TSH is at the upper limits of normal.

The bottom line here is that positive thyroid peroxidase antibodies indicate an autoimmune process; however, they are only a slice of the thyroid pie and do not generally dictate whether or not you need thyroid hormone treatment. Instead, they may influence you and your doctor's decision, amongst other factors like your age, family history, and symptoms.

Overall it's best to talk with your doctor about your unique lab results and symptoms. This way you can ensure you are optimizing your thyroid and overall health.