It is well known that luteinizing hormone (LH), like many other glycoproteins, is heterogeneous and presents several circulating isoforms. Recently, new sensitive immunometric assays measuring intact LH were developed. These assays have been found to underestimate or to be incapable of recognizing LH in some patients. This study was undertaken to determine the prevalence of such cases and to define their clinical characteristics. We compared three LH assays using as capture antibodies either a monoclonal antibody that reacts exclusively with intact LH (ES 600 Boehringer, Stratus Baxter) or a monoclonal antibody against the β subunit of LH (IMX Abbott). In 17% of 90 patients tested, ES 600 measured > 50% lower LH concentrations when compared with the EVIX. Moreover, in two cases LH was not detectable by ES 600 or Stratus, whereas it was normal with the EVIX. We found another five such cases and discuss here the clinical data and results of different hormone measurements in these seven cases of ‘invisible LH'. Although bioactive LH (mouse Leydig cell assay) was normal, the existence of low or even undetectable LH was clinically confusing and led to expensive complementary investigations such as gonadotrophin-releasing hormone analogue tests and magnetic resonance imaging. The uses and limitations of these assays are illustrated by different clinical situations in which the results of the different assays have been misleadin
