THYROID FUNCTION TESTS Thyroid Stimulating Hormone (TSH): The thyroid stimulating hormone (TSH) assay measures the concentration of thyroid stimulating hormone in the serum. TSH assays

The thyroid stimulating hormone (TSH) assay measures the concentration of thyroid stimulating hormone in the serum. TSH assays have been classified by "generation" based on the functional sensitivity of the assay, i.e., the lowest concentration at which the assay is able to maintain an inter-assay precision, expressed in terms of percent coefficient of variation (%CV), of 20% or less. Virtually all TSH assays currently in use are either second or third generation, with functional sensitivities of 0.1 to 0.2 μIU/mL and 0.01 to 0.02 μIU/mL, respectively; thus third generation assays are an order of magnitude more sensitive than second generation assays (sTSH). TSH is recognized as an exquisitely sensitive indicator of thyroid status and thus TSH assays (second or third generation) have been widely adopted as the front-line thyroid function test. In ambulatory patients with intact hypothalamic and pituitary function, a normal TSH result excludes hypoor hyperthyroidism; whereas elevated and suppressed TSH results are diagnostic of hypoand hyperthroidism, respectively. Abnormal TSH results are generally confirmed with a complementary determination of thyroid hormone levels as described below.