<P> In females: ovulation, maintaining of corpus luteum and secretion of progesterone . </P> <P> In males: testosterone secretion . </P> <P> LH supports theca cells in the ovaries that provide androgens and hormonal precursors for estradiol production . At the time of menstruation, FSH initiates follicular growth, specifically affecting granulosa cells . With the rise in estrogens, LH receptors are also expressed on the maturing follicle, which causes it to produce more estradiol . Eventually, when the follicle has fully matured, a spike in 17α - hydroxyprogesterone production by the follicle inhibits the production of estrogens, leading to a decrease in estrogen - mediated negative feedback of GnRH in the hypothalamus, which then stimulates the release of LH from the anterior pituitary . However another theory of the LH peak is a positive feedback mechanism from estradiol . The levels keep rising through the follicular phase and when they reach an unknown threshold, this results in the peak of the LH . This effect is opposite from the usual negative feedback mechanism presented at lower levels . In other words, the mechanism (s) are not yet clear . The increase in LH production only lasts for 24 to 48 hours . This "LH surge" triggers ovulation, thereby not only releasing the egg from the follicle, but also initiating the conversion of the residual follicle into a corpus luteum that, in turn, produces progesterone to prepare the endometrium for a possible implantation . LH is necessary to maintain luteal function for the second two weeks of the menstrual cycle . If pregnancy occurs, LH levels will decrease, and luteal function will instead be maintained by the action of hCG (human chorionic gonadotropin), a hormone very similar to LH but secreted from the new placenta . </P> <P> LH acts upon the Leydig cells of the testis and is regulated by gonadotropin - releasing hormone (GnRH). The Leydig cells produce testosterone (T) under the control of LH, which regulates the expression of the enzyme 17β - hydroxysteroid dehydrogenase that is used to convert androstenedione, the hormone produced by the testes, to testosterone, an androgen that exerts both endocrine activity and intratesticular activity on spermatogenesis . </P>

What effect does an increase in luteinising hormone have on the endometrium
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