[The biological activity of luteinizing hormone (LH)]

In vitro bioassay (BIO) is a very specific and sensitive method for determining levels of luteinizing hormone (LH). It provides a means of evaluating the values obtained with routine radioimmunoassays (RIA). Serum levels with bioassay are higher than those obtained with radioimmunoassay. The LH BIO/LH RIA ratio measures the difference between the two methods and gives a means of evaluating the quality of the LH secreted. The LH BIO/LH RIA ratio is significantly higher at the summit than at the nadir of the LH serum pulses. This ratio also increases after administration of GnRH. The differences in the quality of the LH can be explained by micro-heterogenicity. Immunofluorometric assay of LH (LH FIA) is much more sensitive than either the LH BIO or the LH RIA assays. Unlike the LH BIO/LH RIA ratio, the LH BIO/LH FIA ratio remains unchanged during LH pulses and after administration of GnRH. This difference can be explained by the lack of sensitivity but also by the lack of specificity of the LH RIA. LH RIA is not reliable when the serum levels of LH are low. However, RIA is valid when the serum levels of LH are increased as occurs in polycystic ovary syndromes.