Thyroid Function with Antibodies Blood Test Kit

Thyroid peroxidase (TPO) is an enzyme in the thyroid gland that is essential for producing thyroid hormones—it helps attach iodine to thyroglobulin to make T4 and T3. Anti-TPO antibodies are autoantibodies that attack this enzyme, causing inflammation and gradual destruction of thyroid tissue. This autoimmune attack is the underlying mechanism in Hashimoto's thyroiditis, the most common cause of hypothyroidism in the UK and other iodine-sufficient countries. Anti-TPO antibodies are found in approximately 90% of people with Hashimoto's thyroiditis and about 70% of people with Graves' disease (autoimmune hyperthyroidism). However, approximately 10-15% of the general population have detectable anti-TPO antibodies without currently having thyroid dysfunction—this is called "thyroid autoimmunity" or "positive antibodies with euthyroidism." People with positive anti-TPO antibodies and normal thyroid function have an increased risk (about 2-4% per year) of developing thyroid dysfunction over time, particularly hypothyroidism. Normal is typically <35 IU/mL (or <60 kU/L depending on the assay), with significant elevation usually above 100-500 IU/mL. The level of antibodies doesn't necessarily correlate with the severity of thyroid dysfunction—some people have very high antibodies with mild disease, while others have lower levels but significant problems. Positive anti-TPO antibodies in someone with subclinical hypothyroidism (mildly elevated TSH, normal Free T4) increases the likelihood of progression to overt hypothyroidism. Results outside the normal range may need a follow-up with your GP.

Thyroglobulin (Tg) is a large protein produced by thyroid cells that serves as the scaffold for making thyroid hormones. Iodine is attached to thyroglobulin to produce T4 and T3, which are then released from the thyroglobulin molecule. Anti-thyroglobulin antibodies (anti-Tg or TgAb) are autoantibodies that target this protein. Like anti-TPO antibodies, they indicate autoimmune thyroid disease. Anti-Tg antibodies are found in approximately 60-85% of people with Hashimoto's thyroiditis and 30-60% of people with Graves' disease. They are generally considered less sensitive and specific than anti-TPO antibodies for diagnosing autoimmune thyroid disease. However, about 3-5% of people with autoimmune thyroiditis have only anti-Tg antibodies without anti-TPO antibodies, so testing both increases diagnostic sensitivity. Anti-Tg antibodies are also important in thyroid cancer follow-up, where they can interfere with thyroglobulin tumour marker measurements. Normal is typically <40 IU/mL (or <115 kU/L depending on the assay). Like anti-TPO, the presence of anti-Tg antibodies in someone with normal thyroid function indicates increased risk of future thyroid dysfunction and warrants periodic monitoring. If both antibody tests are positive, this strongly suggests autoimmune thyroid disease. If both are negative in someone with abnormal thyroid function, the cause is likely not autoimmune (for example, iodine deficiency, medication-induced, or nodular thyroid disease). Results outside the normal range may need a follow-up with your GP.

Free T3 measures the unbound, active fraction of triiodothyronine. T3 is the most metabolically active thyroid hormone—about 3-4 times more potent than T4. Most T3 in the body (approximately 80%) is produced by conversion of T4 to T3 in peripheral tissues (liver, kidney, muscles, brain) by deiodinase enzymes, rather than being directly secreted by the thyroid. T3 is the hormone that actually enters cells and exerts thyroid effects on metabolism. Free T3 provides additional information beyond TSH and Free T4. It can be particularly useful in hyperthyroidism (where Free T3 may be elevated earlier or more prominently than Free T4), in assessing T4-to-T3 conversion (some people convert poorly), and in monitoring patients on T3-containing thyroid preparations. In hypothyroidism, Free T3 often remains normal until the condition is quite advanced because the body prioritises T3 production. Normal Free T3 is typically 3.5-6.5 pmol/L. Low Free T3 with normal TSH and Free T4 can occur in non-thyroidal illness ("sick euthyroid syndrome"), starvation, or certain medications—this is usually not a primary thyroid problem. Elevated Free T3 with suppressed TSH is seen in hyperthyroidism. Some patients feel T3 levels are important for their wellbeing, though this is debated in medical literature. Results outside the normal range may need a follow-up with your GP.