From the Guidelines
TPO and TSI are two distinct thyroid antibody tests utilized to diagnose autoimmune thyroid disorders, with TPO detecting antibodies against thyroid peroxidase and TSI measuring antibodies that mimic thyroid-stimulating hormone (TSH).
Definition of TPO and TSI
- TPO (Thyroid Peroxidase Antibodies): This test detects antibodies against the enzyme thyroid peroxidase, which is involved in thyroid hormone production. High TPO levels are commonly associated with Hashimoto's thyroiditis, an autoimmune condition causing hypothyroidism 1.
- TSI (Thyroid Stimulating Immunoglobulin): This test measures antibodies that mimic thyroid-stimulating hormone (TSH), leading to excessive thyroid hormone production. Elevated TSI levels are typically seen in Graves' disease, an autoimmune disorder causing hyperthyroidism 1.
Clinical Significance
These tests are usually ordered together with other thyroid function tests (TSH, T3, T4) to provide a comprehensive assessment of thyroid health. Results should be interpreted by a healthcare provider in the context of clinical symptoms and other test results. If abnormal, treatment may involve thyroid hormone replacement or anti-thyroid medications, depending on the specific condition diagnosed. According to the most recent study 1, autoimmune thyroid disease is the most common autoimmune disorder associated with diabetes, occurring in 17–30% of individuals with type 1 diabetes.
Diagnostic Approach
The presence of thyroid autoantibodies, such as TPO, is predictive of thyroid dysfunction, most commonly hypothyroidism, although hyperthyroidism occurs in a smaller percentage of patients with type 1 diabetes 1. Thyroid function tests may be misleading if performed at the time of diagnosis due to the effect of previous hyperglycemia, ketosis, or ketoacidosis, weight loss, etc. Therefore, if performed at diagnosis and slightly abnormal, thyroid function tests should be repeated soon after a period of metabolic stability and achievement of glycemic targets.
From the Research
Thyroid Peroxidase (TPO)
- Thyroid peroxidase (TPO) is an enzyme that participates in thyroid hormone biosynthesis 2
- TPO is also a major autoantigen in autoimmune thyroid diseases (AITD) 2, 3, 4
- TPO autoantibodies are frequently present in euthyroid subjects, with a prevalence of 12-26% 3
- The presence of TPO autoantibodies can herald impending thyroid failure, even in subjects with normal thyrotropin (TSH) levels 3
- Measuring TPO antibodies in euthyroid subjects can be used to identify subjects with increased risk for hypothyroidism 3
Thyroid-Stimulating Immunoglobulin (TSI)
- There is no direct information available on Thyroid-Stimulating Immunoglobulin (TSI) in the provided studies
Relationship between TPO and Thyroid Function
- The levels of T3, T4, and TSH in individuals with normal and raised anti-TPO antibody titers are significantly different 5
- A correlation between TSH and T4 levels and abnormal anti-TPO antibody has been detected 5
- The presence of TPO autoantibodies can be used to identify subjects with increased risk for hypothyroidism, and to predict thyroid dysfunction in certain populations 3, 5