This document discusses thyroid hormone tests (T3, T4, TSH, fT4) and their principles, procedures, and clinical significance. It describes the hormones T3 and T4, how they are regulated by the hypothalamus-pituitary-thyroid axis, and common thyroid disorders like hypothyroidism and hyperthyroidism. It provides details on specific assays for the hormones, including radioimmunoassay, immunoradiometric assay, enzyme immunoassay, and electrochemiluminescent assay. Reference ranges and clinical implications of test results are also covered.