Thyroid peroxidase (TPO) antibodies are specific proteins produced by the immune system that target an enzyme found within the thyroid gland. This enzyme, thyroid peroxidase, plays a significant part in the production of thyroid hormones. Their presence indicates an immune system response directed toward the thyroid.
What Are Thyroid Peroxidase Antibodies?
Thyroid peroxidase (TPO) is an enzyme located in the thyroid gland, a butterfly-shaped organ situated at the base of the neck, below the Adam’s apple. This enzyme is integral to the synthesis of thyroid hormones, specifically by aiding in the conversion of thyroid hormone T4 to T3 and facilitating the binding of iodine to proteins within the gland. Thyroid hormones, such as T3 and T4, are important for regulating various bodily processes, including metabolism, growth, and development.
TPO antibodies are proteins generated by the immune system that mistakenly identify thyroid peroxidase as a harmful substance. These antibodies then attack the TPO enzyme, which can disrupt the normal production of thyroid hormones. The presence of TPO antibodies in the bloodstream signifies an autoimmune response, where the body’s own defense system targets its thyroid tissue.
Understanding the Normal Range
The “normal” or reference range for thyroid peroxidase (TPO) antibodies varies among different laboratories, so it’s important to consult the specific reference values provided with your test results. Generally, a typical range for TPO antibodies is considered to be less than 34 international units per milliliter (IU/mL). Some laboratories may define their normal threshold as less than 35 IU/mL or even less than 5.6 IU/mL.
A TPO antibody level within this normal range suggests no significant autoimmune activity directed against the thyroid gland. While a low or undetectable level is considered optimal, it indicates levels below a clinically significant threshold, not necessarily the complete absence of antibodies.
Significance of Elevated Levels
When TPO antibody levels are above the normal reference range, it indicates the presence of these autoantibodies and an ongoing autoimmune process attacking the thyroid gland. This can impact its normal function and lead to disruptions in thyroid hormone production.
Elevated TPO antibodies are associated with two primary autoimmune thyroid conditions. The most common is Hashimoto’s thyroiditis, a chronic inflammatory condition of the thyroid gland that often leads to hypothyroidism, or an underactive thyroid. Less commonly, elevated TPO antibodies can also be observed in Graves’ disease, which causes hyperthyroidism, or an overactive thyroid. In conditions like Hashimoto’s, these antibodies contribute to the gradual destruction of thyroid cells, impairing the gland’s ability to produce sufficient hormones.