| Follicle-stimulating hormone and luteinizing hormone
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| Although FSH and LH have been given their names on the basis of their function in the female, it is now clear that identical hormones are secreted and function in the male
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| Both FSH and LH are secreted in the male and female and appear to be under the influence of the same stimulus (GnRH). There is growing evidence that alterations in the GnRH pulse frequency and amplitude can influence the relative amounts of FSH and LH secreted by the gonadotroph. Inhibin, as a selective feedback inhibitor of FSH, also affects the relative output of FSH and LH from the cell.
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| FSH and LH are both glycoproteins with molecular weights of approximately 28 kDa. Each contains an identical α-subunit (shared with TSH) and a specific β-subunit. The gene for LH has recently undergone duplication to produce HCG, a gonadotropin secreted by embryonic tissues (Fig. 37.8).
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Although GnRH is essential for the secretion of intact FSH and LH, feedback from estradiol and testosterone plus gonadal peptides such as inhibin have a secondary effect. Feedback by estradiol is especially interesting because it may have either negative or positive effects on gonadotropins depending on the stage of the menstrual cycle. An outline of the control of the hypothalamo-pituitary-gonadal axes, both for adult men and women, is shown in Figure 37.9.
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| Figure 37.8 Relative structures of glycoprotein hormones. The three pituitary glycoprotein hormones (TSH, FSH, LH) and the placental glycoprotein (HCG) share a common α-subunit. Hormone specificity is conveyed by the β-subunit and the resulting three-dimensional protein structure. CHO indicates the approximate location of carbohydrate side chains. HCG differs from LH solely by having an additional 32 amino acids in its β-subunit. aa, amino acids. |
| The production rate of LH in men and ovulating women is about 200 IU/day; the corresponding figure for FSH is around 50 IU/day. These production rates are greatly increased in postmenopausal women in whom there is no negative feedback from ovarian steroids. The half-life of LH in plasma is approximately 50 minutes, FSH has a longer half-life of about
4 hours. Both FSH and LH concentrations vary considerably depending on age and sex (Fig. 37.10).
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