How to identify Thyroid dysfunction causes ?

Test to identify cause of thyroid dysfunction

a. Antibodies to thyroid peroxidase
These antibodies are found in almost 95% patients with autoimmune hypothyroidism secondary to Hashimoto’s thyroiditis early in the course of the disease and in some patients with other autoimmune thyroid disease. Their target antigen is thyroid peroxidase enzyme. These antibodies can fix the complement and play a major pathogenic role in autoimmune thyroiditis leading to impaired formation of T3 and T4.

b. Antibodies to thyroglobulin
This occurs with lower frequency and do not fix complement and are not known to play a direct pathogenic role in aetiology of autoimmune thyroid disease in man. These antibodies can interfere in Tg measurement giving falsely decreased concentration in IMA. The main reason to measure them to indicate possible assay interference in Tg assay.

c. Antibodies to TSH receptor
This is called Grave’s disease where autoantibodies (IgG) against TSH receptor are formed and these antibodies mimic the function of TSH leading to overt hyperthyroidism, in this condition the TSH level is reduced. There is thyroid gland Hyperfunction. Other TSH receptor antibodies (blocking antibodies) although infrequently encountered can inhibit gland function and lead to hypothyroidism.

Opthalmopathy in Grave’s disease is poorly understood. Orbital muscle, connective tissue and adipose tissue become infiltrated with lymphocytes and macrophages. The extracellular compartment of extraocular muscle and orbital fibro-adipose tissue becomes oedematous owing to water deposition caused by production of glycosaminoglycans by orbital fibroblasts. TSH receptor is also expressed in orbital connective tissue, orbital fat and extraocular muscle fibers which are also the target of these autoantibodies.

99mTc given intravenously as pertechneate, is concentrated within the gland but not organified into thyroid hormones and therefore diffuses out of the gland with time, and the functioning thyroid tissue can be obtained by imaging technique and determining the total dose taken per time, activity of gland can be determined.

Use of 123I being best isotope to use for imaging thyroid tissue, this provides the idea on Organification process. Here radioactive isotope of iodine is given and proportion of total administered dose of isotoe concentrated within the thyroid gland during a given time period is quantified to estimate the activity of gland.