Hormones of thyroid gland. Evaluation of laboratory data and correlations in thyroid diseases.
measure either total amount of hormone (bound plus unbound) or just the free portion
- T4 and T3 are hormones produced by the thyroid gland. They help control the rate at which the body uses energy, and are regulated by a feedback system.
- TSH from the pituitary gland stimulates the production and release of T4 (primarily) and T3 by the thyroid.
- Most of the T4 and T3 circulate in the blood bound to protein. A small percentage is free (not bound) and is the biologically active form of the hormones.
- Thyroid autoantibodies are antibodies that develop when a person's immune system mistakenly targets components of the thyroid gland or thyroid proteins, leading to chronic inflammation of the thyroid (thyroiditis), tissue damage, and/or disruption of thyroid function.
- Tests detect the presence and measure the quantity of specific thyroid autoantibodies.
- When thyroid antibodies interfere with this process, they can lead to chronic conditions and autoimmune disorders associated with hypothyroidism or hyperthyroidism, such as Graves disease or Hashimoto thyroiditis.
Tests to diagnosis autoimmune thyroid disorder:
- Thyroperoxidase antibody (TPO)—a marker for autoimmune thyroid disease; it can be detected in Graves disease or Hashimotos thyroiditis.
- Thyroglobulin antibody (TGAb)—also a marker, this antibody targets thyroglobulin, the storage form of thyroid hormones.
- Thyroid stimulating hormone receptor antibodies (TSHRAb)—includes two types of autoantibodies that bind to TSH receptors in the thyroid: Thyroid stimulating immunoglobulin (TSI) binds to receptors and promotes the production of thyroid hormones, leading to hyperthyroidism.
- Thyroid binding inhibitory immunoglobulin (TBII) blocks TSH from binding to receptors, blocking production of thyroid hormones and resulting in hypothyroidism. TBII is not routinely tested, but TSI is often used to help diagnose Graves disease.
- Indications: Clinical doubts for thyroid dysfunction. Monitoring of individual treatment in already diagnosed thyroid dysfunction
- Sources of error
- Drugs: (increasing fT3 and fT4 levels) carbamazepine, lithium salts
- Drugs (decreasing fT3 and fT4 levels) androgens, dopamin, corticosteroids
- Salicylic acid – disturb the tissue binding of thyroid hormones
- Autoantibodies – interfere with immunometric methods and lead to false results for TSH, fT4 and fT3
- Increased serum total protein – false (+)
- Decreased total serum protein – false (-)
Thyroid panel
The thyroid panel usually includes:
TSH (thyroid-stimulating hormone) – to test for hypothyroidism, hyperthyroidism and to monitor treatment for a thyroid disorder
Free T4 (thyroxine) – to test for hypothyroidism and hyperthyroidism; may also be used to monitor treatment