Serum Bioactive Follicle-Stimulating Hormone-Like Activity Increases during Pregnancy*

Abstract

It is presently accepted that high circulating steroid levels during pregnancy suppress pituitary immunoreactive FSH (I-FSH) secretion from early pregnancy to term. In this study we tested the hypothesis that bioactive FSH (B-FSH) in the sera of pregnant women would likewise be suppressed. Serum samples were obtained from one woman daily for 21 days after the I-LH surge, and in a second, every 10 min for 10 h on days 10, 20, and 27 after the I-LH surge. Single serum samples were also obtained from women at 2-41 weeks of gestation. In all samples, I-LH/hCG, I-FSH, B-FSH, progesterone (P), and estradiol (E2) were measured. In the first woman, serum I-FSH levels were suppressed from days 9-21 after the LH surge (0.2 +/- 0.04 ng/mL; 1.9 +/- 0.3 IU/L), whereas serum B-FSH increased linearly from 1.7 ng/mL (5.3 IU/L) on day 9 post-I-LH surge to 6.0 ng/mL (18.6 IU/L) on day 17 (r = 0.91; P less than 0.01) and remained elevated through day 21. In the second woman, the mean +/- SE of the serum I-FSH concentrations were 0.7 +/- 0.04, 0.1 +/- 0.01, and 0.1 +/- 0.01 ng/mL (5.5 +/- 0.3, 0.9 +/- 0.1, 1.1 +/- 0.1 IU/L) on days 10, 20, and 27 post-I-LH surge, respectively. The mean serum B-FSH concentrations increased from 3.1 +/- 0.2 ng/mL (9.6 +/- 0.7 IU/L) on day 10 to 7.5 +/- 0.6 ng/mL (23.2 +/- 1.7 IU/L) on day 27. Using the DETECT pulse analysis method at the 0.01 confidence level, the serum I-FSH pattern was relatively nonpulsate, while serum B-FSH averaged 8, 7, and 10 pulses/10 h on post-LH surge days 10, 20, and 27. Incremental peak amplitudes were 3.5 +/- 0.7, 4.6 +/- 0.4, and 8.0 +/- 1.3 ng/mL (10.9 +/- 2.2, 14.3 +/- 1.2, and 24.8 +/- 4.0 IU/L), respectively. In the cross-sectional study, serum I-FSH levels remained low, while serum B-FSH concentrations increased from 4.2 +/- 0.8 ng/mL (13.2 +/- 2.5 IU/L) during the early first trimester to 59.3 +/- 2.9 ng/mL (183.8 +/- 9.0 IU/L) by the late third trimester. There was no correlation between serum B-FSH and hCG levels. Incubates of term placentae secreted B-FSH in preference to I-FSH (i.e. B-FSH, 33.0 +/- 9.6 ng/h/g placental tissue; I-FSH, 0.7 +/- 0.1 ng/h/g).(ABSTRACT TRUNCATED AT 400 WORDS)