Serum Bioactive Follicle-Stimulating Hormone during the Human Menstrual Cycle and in Hyper- and Hypogonadotropic States: Application of a Sensitive Granulosa Cell Aromatase Bioassay*

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Abstract
A sensitive in vitro assay based on the stimulation of estrogen production by cultured rat granulosa cells was recently developed for the measurement of biologically active FSH. This bioassay system is specific for FSH, highly sensitive, and capable of measuring basal FSH levels in rat serum. The granulosa cell aromatase bioassay was improved by the use of additives known to enhance FSH activity and by pretreatment of serum with 12% polyethylene glycol to remove inhibitory substances. We applied this method to the measurement of bioactive FSH levels in serum samples from human subjects. As determined in daily blood samples during ovulatory menstrual cycles in seven women, bioactive FSH levels exhibited a pattern closely resemblipg that of immunoreactive FSH. The mean bioactive serum FSH levels were 29.9, 20.5, 39.2, and 14.8 mlU/ml for the early follicular phase, late follicular phase, preovula-tory surge, and luteal phase, respectively. The bio- to immuno-ratio (B:I) throughout the menstrual cycle ranged from 1.4-3.4, with a mean of 2.5. The ratios for early follicular phase, late follicular phase, preovulatory surge, and luteal phase were 2.7, 2.3, 1.4, and 2.6, respectively. The correlation coefficient (r) of the serum FSH values obtained by bioassay and RIA was 0.91. FSH bioactivity was also measured in patients in each of the following categories with the following mean values: oral contraceptive pill users (undetectable), hypothalamic amenorrhea (18.7 mlU/ml; B:I, 2.6), premature ovarian failure (163 mlU/ml; B:I, 1.7), and postmenopausal women (191 mlU/ml; B:I, 1.6). These findings suggest that measurement of immunoreactive FSH levels correctly reflects the biological activity of FSH in serum of cycling women and patients in certain hyper- and hypogona-dotropic states. The granulosa cell aromatase bioassay represents a new tool for future assessments of biologically active FSH in physiological and pathophysiological conditions.