Explain the principle of radioimmunoassay (RIA) and outline its role in the diagnosis of thyroid disorders. Add a note on the advantages and disadvantages of this technique.
Principle of Radioimmunoassay (RIA):
RIA is a highly sensitive immunoassay based on competitive binding between a radiolabeled antigen and an unlabeled antigen for limited antibody sites. A fixed amount of antibody is mixed with radiolabeled antigen. When the patient’s unlabeled antigen is added, it competes for antibody binding. As unlabeled antigen concentration rises, less radiolabeled antigen binds. Bound and free antigens are separated, and radioactivity is measured. A standard curve of known antigen concentrations allows precise determination of unknown sample levels.
Role in Thyroid Diagnosis:
RIA accurately measures TSH, T3, T4, and thyroid autoantibodies (anti-TPO, anti-Tg). Elevated TSH indicates hypothyroidism, low TSH suggests hyperthyroidism, while T3/T4 levels assess thyroid gland function. Detection of autoantibodies helps diagnose Hashimoto’s thyroiditis or Graves’ disease.
Advantages: High sensitivity, specificity, and versatility for diverse analytes. Disadvantages: Requires radioisotopes, short reagent shelf-life, specialized equipment, cost, and strict regulatory compliance.
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