First-generation thyroid-stimulating hormone (TSH) radioimmunoassays (RIAs) had detection limits of approximately 1 mU/L. Since the normal range for serum TSH is approximately 0.4 to 5.0 mU/L. These assays were useful for the diagnosis of primary hypothyroidism (in which serum TSH concentrations are appropriately elevated) but were not sufficiently sensitive to distinguish between normal serum TSH concentrations and the low serum TSH concentrations present in most patients with hyperthyroidism.
Second-generation TSH immunometric assays have detection limits of approximately 0.1 mU/L. Since this detection limit is just below the normal range for TSH of approximately 0.4 to 5.0 mU/L, these assays can be used as screening tests to distinguish hyperthyroidism from euthyroidism and hypothyroidism.However, since the range of subnormal TSH measurement is very limited, values near or at the…
