From the Guidelines
TPO, or thyroid peroxidase, is an enzyme crucial for thyroid hormone production, and a TPO test measures antibodies against this enzyme in the blood, which can indicate autoimmune thyroid disorders like Hashimoto's thyroiditis.
Definition and Purpose of TPO Test
The TPO test is a simple blood draw, usually done alongside other thyroid function tests like TSH, T3, and T4, with no special preparation needed.
- Normal TPO antibody levels are generally below 35 IU/mL, though ranges can vary by lab.
- High TPO antibody levels suggest the immune system may be attacking the thyroid gland, but this doesn't always mean a thyroid disorder is present - some people with elevated antibodies have normal thyroid function.
Importance of TPO Antibodies
Understanding TPO antibodies is important because they can help diagnose thyroid issues before changes in thyroid hormone levels occur, allowing for earlier intervention if needed, as indicated by a study from Sweden that found antithyroid peroxidase antibodies were more predictive than antithyroglobulin antibodies in multivariate analysis 1.
Clinical Implications
Thyroid function tests may be misleading if performed at the time of diagnosis due to previous hyperglycemia, ketosis, or ketoacidosis, and should be repeated after a period of metabolic stability and achievement of glycemic targets, according to the standards of care in diabetes-2024 1.
- Subclinical hypothyroidism may be associated with an increased risk of symptomatic hypoglycemia and reduced linear growth rate.
- Hyperthyroidism alters glucose metabolism and usually causes deterioration of glycemic control.
Recommendations
The presence of thyroid autoantibodies, including TPO, is predictive of thyroid dysfunction, and patients with type 1 diabetes should be screened for these antibodies soon after diagnosis, as recommended by the standards of care in diabetes-2024 1.
- Thyroid-stimulating hormone concentrations should be measured at diagnosis when clinically stable or soon after glycemic control has been established, and rechecked every 1-2 years or sooner if symptoms or signs suggestive of thyroid dysfunction develop.
From the Research
Definition and Function of Thyroid Peroxidase (TPO)
- Thyroid peroxidase (TPO) is an enzyme that participates in thyroid hormone biosynthesis 2, 3, 4, 5, 6.
- TPO is a key enzyme in the formation of thyroid hormones and a major autoantigen in autoimmune thyroid diseases (AITD) 2, 3, 5.
Structure and Properties of TPO
- TPO is a dimeric membrane-bound enzyme of thyroid follicular cells, responsible for thyroid hormone biosynthesis 6.
- TPO has two active sites, making it an unusual enzyme 5, 6.
- The propeptide sequence of TPO has a redundant role in its cellular, enzymatic, and immunological activity 6.
Role of TPO in Autoimmune Thyroid Diseases
- TPO is a major autoantigen in autoimmune thyroid diseases (AITD) 2, 3, 5.
- Autoantibodies to TPO (TPOAbs) are a hallmark of AITD 5.
- TPOAbs are directed predominantly to two immunodominant determinants (IDR) termed IDR-A and IDR-B regions, with the latter antibodies more predominant in autoimmune disease 5.
Clinical Utilities of TPO Autoantibody Testing
- Measuring TPO antibodies in euthyroid subjects can be used to identify subjects with increased risk for hypothyroidism 3.
- TPO autoantibody testing can be used to predict first trimester hypothyroidism, and postpartum thyroid dysfunction in women who wish to become pregnant and those with an increased risk per se who are pregnant 3.