Thyrotropin/Thyroid Stimulating Hormone (TSH) Measurement

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Introduction

For more than twenty-five years, TSH methods have been able to detect the TSH elevations that are characteristic of primary hypothyroidism. Modern-day TSH methods however, with their enhanced sensitivity are also capable of detecting the low TSH values typical of hyperthyroidism. These new methods are often based on non-isotopic immunometric assay (IMA) principles and are available on a variety of automated immunoassay analyzer platforms. Most of the current methods are capable of achieving a functional sensitivity of 0.02mIU/L or less, which is the necessary sensitivity for detecting the full range of TSH values observed between hypo-and hyperthyroidism. With this level of sensitivity, it is possible to distinguish the profound TSH suppression typical of severe Graves' thyrotoxicosis (TSH < 0.01 mIU/L) from lesser degrees of TSH suppression (0.01 - 0.1 mIU/L) observed with mild (subclinical) hyperthyroidism and in some patients with a non-thyroidal illness (NTI).

In the last decade the diagnostic strategy for using TSH measurements has changed as a result of the sensitivity improvements in these assays. It is now recognized that TSH measurement is a more sensitive test than FT4 for detecting both hypo-and hyperthyroidism. As a result, some countries now promote a TSH-first strategy for diagnosing thyroid dysfunction in ambulatory patients (provided that the TSH method has a functional sensitivity ≤ 0.02 mIU/L). Other countries still favor the TSH + FT4 panel approach, because the TSH-first strategy can miss patients with central hypothyroidism [Section-3 C4(f)] or TSH-secreting pituitary tumors [Section-3 C4(g)i].[19,195,196,197] An additional disadvantage of the TSH-centered strategy is that the TSH-FT4 relationship cannot be used as a "sanity check" for interferences or detection of unusual conditions characterized by discordance in the TSH/FT4 relationship ( Table 1 ).

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