The upper limit of normal for thyrotropin is 3 or 4 milli-int. units/L

Recently Musto et al. (Clin Chem 30: 329-330, 1984) noted that, despite the clinical importance of accurate measurement of thyrotropin at low concentrations, the upper limit of normal is well below the stated upper limit of normal of most commercial tests. Here I amplify their statement.