THYROID

Thyroglobulin Antibody (TgAb) & Thyroid Peroxidase Antibody (TPOAb)

Antibodies to thyroglobulin (TgAb) and thyroid peroxidase (TPOAb) occur in many different thyroid diseases and in some healthy individuals. The measurement of both TgAb and TPOAb can be of value in the diagnosis of autoimmune thyroiditis (Hashimoto's Disease), Graves' Disease, endemic goiter, and subacute thyroiditis. These measurements, when used as part of a thyroid autoimmune diagnostic profile (including TRAb) can be useful in the differential diagnosis and management of thyroid disease. Assessment of Tg and TPO antibodies may also be of value in the following disease diagnoses: Addison's Disease, Type 1 diabetes mellitus, polyendocrine autoimmunopathies, members of families prone to organ specific autoimmunity, and thyroid autoimmunity in pregnancy.

TSH Receptor Antibody (TRAb)

There is convincing evidence that antibodies to the TSH receptor (TRAb) are responsible for Graves' hyperthyroidism. These antibodies are detectable in approximately 90% of untreated Graves' patients when measured by receptor assays. The presence of TRAb indicates that the patient's thyrotoxicosis is of autoimmune etiology rather than due to toxic nodular goiter. Because the form of treatment for Graves' disease may differ from the treatment of other forms of thyrotoxicosis, an initial TRAb measurement is clearly of value.

Additionally, TRAb levels tend to fall during antithyroid drug treatment for Graves' disease. The absence of antibodies after a course of drug therapy may indicate disease remission, and the withdrawal of therapy can be considered. If TRAb are still present after a course of antithyroid drugs, the risk of relapse is high, and surgery or radioiodine therapy can be considered.

The measurement of TRAb by receptor assay provides a rapid, sensitive, specific and inexpensive diagnostic marker for Graves' disease.

Serum Thyroglobulin

Thyroglobulin (Tg) is a glycoprotein (approximately 650,000 Daltons) synthesized and stored in the follicular colloid of the thyroid gland. It functions as a prohormone in the synthesis of thyroxine (T4) and triiodothyronine (T3). 1Newly synthesized Tg is transported by a vesicle to which is bound thyroid microsomal antigen, an organ specific protein shown in recent studies to be the enzyme thyroid peroxidase (TPO).2

Thyroglobulin is the principal thyroid antigen, produced only by thyroid derived tissues. 3,4,5 A small quantity of Tg (0-60 ng/mL) is continuously released into the serum of healthy, thyroid-intact individuals and is detectable by sensitive methods.6 These serum Tg levels are highly variable between individuals and serve no established physiological or diagnostic function.

As functioning thyroid tissue is the only source of circulating thyroglobulin (Tg), the measurement of this prohormone in thyroidectomized individuals (via surgery and with or without radioiodine ablation) is useful clinically as an aid in the detection of local and metastatic thyroid tissue. 11-14 Accordingly, serum Tg determinations are often widely used to complement other diagnostic aids, such as whole-body scan (WBS), computed tomography, high-resolution ultrasound, and chest x-ray, to detect the presence of functional thyroid tissue or tumor.