Antithyroid antibodies are autoantibodies that target one or more components of the thyroid gland and can act as markers to help diagnose autoimmune thyroid conditions. For example, thyroid stimulating hormone receptor antibodies (TRAb) are seen mostly in Graves' disease, while thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb) are typical for Hashimoto's thyroiditis (but may also be present in Graves' disease). However, elevated thyroid antibodies do not always indicate disease, as somewhat increased levels may also be present in healthy individuals.
Percentage of cases associated with positive autoantibodies for various conditions:
|TSH receptor antibodies (TRAb)||Thyroid peroxidase antibodies (TPOAb)||Thyroglobulin antibodies (TgAb)|
|∼ 90%||∼ 70%||∼ 50–70%|
|∼ 10–15%||> 90%||> 80%|
|No association||Sporadic||∼ 25% (important for follow-up!)|
|Other conditions|| |
∼ 15% in multinodular goiter
|> 60% in postpartum thyroiditis||∼ 40% in other autoimmune diseases (eg, type 1 diabetes mellitus)|
|General population||Negative||∼ 5%||∼ 5%|
TSH receptor antibodies (TRAbs): There are three types of antibodies that may have either a stimulating (most common), blocking, or neutral effect on the TSH receptors.
- In Graves disease: stimulating TRAbs (thyroid stimulating immunoglobulin, TSI) → ↑ thyroid function and growth → hyperthyroidism and diffuse goiter
- In Hashimoto thyroiditis: blocking TRAbs → competitively block the activity of TSH on the receptor → hypothyroidism
- Neutral TRAbs have no effect on the thyrotropin receptors and their clinical and physiological relevance remains unclear.
Thyroid peroxidase antibodies (TPOAb)
- TPO functions as a catalyst to the organification and coupling reactions in thyroxine production. TPOAbs prevent it from doing so.
- In Hashimoto thyroiditis: TPOAbs cause a fall in thyroid hormone production → hypothyroidism
- In Graves disease: TPOAbs have a complement fixing and cytotoxic capacity and are not known to play a role in pathogenesis of Graves' disease, but are seen as a result of the autoimmune nature of the disease.
Thyroglobulin antibodies (TgAb)
- Thyroglobulin (Tg) is a protein produced in thyroid cells, which is involved in the synthesis of thyroid hormone and is normally found only in low concentrations in the bloodstream.
- Destructive thyroid processes (i.e., Hashimoto thyroiditis; ) and/or rapid disordered growth of thyroid tissue (e.g., Graves disease, follicular thyroid cancer) → release of circulating free Tg into the bloodstream → TgAb induction