Luteinizing Hormone

Introduction

Gonadotropins are heterodimeric peptide hormones that are released by the hypothalamus and pituitary gland. Gonadotropin hormones include Follicle stimulating hormone (FSH), Luteinizing hormone (LH), Chorionic gonadotropin (hCG), and Gonadotropin-releasing hormone (GnRH). FSH and LH are secreted by the anterior pituitary gland while hCG is produced by the placenta. GnRH is the hormone that significantly controls the level of FSH and LH, and is produced by the hypothalamus. These hormones except GnRH consist of the dipeptide chain- alpha and beta. The Alpha chain in all three hormones shares homogeneity while the beta chain is unique having specific receptivity.

Follicle-stimulating Hormone

FSH is secreted by the anterior pituitary lobe. In females, it interacts with the receptors present in the ovarian follicle and promotes follicular proliferation. FSH works in collaboration with LH to induce ovulation during the menstrual cycle. In males, FSH play important role in spermatogenesis and other.

Human Chorionic Gonadotropin

HCG is secreted by the placenta, and formed after the implantation of an embryo. The elevated levels of hCG in blood or urine samples ensure pregnancy. It interacts with LH receptors present in the ovary and maintains the structure of the corpus luteum which secretes progesterone, essential to maintaining pregnancy.

Gonadotropin-releasing Hormone

GnRH is a neurohormone secreted by the hypothalamic gland. It is a decapeptide hormone that acts on the pituitary gland and influences it to release FSH and LH. The GnRH activity increases markedly at the time of puberty.

Luteinizing Hormone

LH is secreted by the anterior pituitary gland in response to hypothalamic secretion, GnRH. LH surge at mid of the menstrual cycle causes ovulation and also maintains the level of progesterone if a fertilized egg gets implanted in the uterus. In males elevated levels of LH cause the production of testosterone by Leydig cells.

Structure of Luteinizing Hormone

• Luteinizing hormone is a protein-based hormone composed of two polypeptide chains.

• The two chains-alpha and beta differ in composition, number, and kind of amino acid residue. The two chains are linked by a non-covalent bond.

• The alpha subunit shows homogeneity and is similar to other gonadotropin hormones like FSH, LH, and hCG. In humans, Lutropin is composed of 92 amino acids whereas in other vertebrates the alpha chain has 96 amino acid residues.

• The beta subunits are distinct in each gonadotropin hormone. The beta subunit of LH hormone has 120 amino acid residues. This confers specificity to LH hormone as it interacts with specific LH receptors present in gonadotropin cells.

• The half-life of the LH hormone, meaning the time taken by the hormone to degrade its activity is 20 minutes.

Function of LH in Males

In males, LH hormone act on LH/choriogonadotropin receptor (LHCGR) present on the surface of (plasma membrane) of Leydig cells. The receptor is a G-protein coupled receptor that on the reception of signal starts producing the male sex hormone, testosterone. Testosterone is responsible for spermatogenesis and by negative feedback mechanism regulates the level of LH secreted by the anterior pituitary gland.

• Gonadotropin-releasing hormone is produced by the hypothalamus gland. This hormone targets the anterior pituitary and triggers it to produce LH.

• In turn, the LH hormone targets Leydig cells and stimulates them to secrete testosterone.

• Similarly, FSH hormone from the anterior pituitary targets Sertoli cells of seminiferous tubules and produces Androgen Binding Protein (ABP) and inhibin.

• The synergistic effect of both hormones causes spermatogenesis.

• After reaching the threshold level the increased testosterone concentration kicks off the negative feedback mechanism.

• This decreases the secretion of LH and FSH by the anterior pituitary thereby terminating the production of testosterone.

Function of LH in Females

• During the menstruation cycle, the FSH hormone causes the proliferation of ovarian follicles.

• As the follicles mature they start expressing LH receptors on their plasma membrane.

• Eventually, there is a rise in both FSH and LH levels. As the maxima are reached, known as LH surge it triggers ovulation (release of ova).

• LH surge is also responsible to transform residual Graffian follicle to Corpus luteum.

• The corpus luteum produces progesterone, the pregnancy hormone which prepares uterine endometrium for implantation.

• If pregnancy occurs the level of LH decreases and the corpus luteum is maintained by the hCG hormone which is similar to LH.

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Trends of LH

During the menstrual cycle, the level of LH is lowest during the early follicular phase. As the cycle progress, there is a gradual increase in the level of LH. In the mid of the cycle, the level of LH increases to the maximum causing ovulation. During mid-cycle, the LH level found in the blood measures 6.17 to 17.2 IU/L. In the luteal phase, there is a decrease in LH level causing progesterone to increase.

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Disorders of LH

• If there is an elevated level of LH in the body this signifies malfunctioning of sex organs as they do not produce normal levels of steroid hormone.

• The high levels of LH are responsible to cause Turner syndrome or Fragile X in females while in males the high level cause Klinefelter syndrome.

• In the case of mild level fluctuation of LH in females, it results in polycystic ovary syndrome (PCOS). Women suffering from such conditions show irregular periods and infertility.

• The low level of LH in humans causes delayed sexual development as a low LH level reduces the activity of the pituitary gland.

• Low-level results in rare conditions of Kallmann syndrome; insufficient testosterone and estrogen production.

• Functional Hypothalamic Amenorrhea resulting in missed or irregular periods is a result of underproduction of LH levels.

• If LH deficiency is untreated before the onset of puberty it causes delayed or no puberty. The secondary sexual characteristics are not developed in males and females leading to infertility.

Conclusion

Luteinizing hormone is a gonadotropin hormone released by the anterior pituitary gland. The levels of LH are regulated by the hypothalamus gland as it secretes GnRH. Luteinizing hormone is responsible for spermatogenesis in males and ovulation in females. Thus, it is one of the essential hormones in sexual and reproductive development in both males and females.

FAQs

Q1. What is the LH test?

Ans. LH test is a normal blood test to measure the level of LH in the human body. These tests in women diagnose the condition of infertility, ovulation period, and the cause of irregular menses. In males, these tests help in diagnosing infertility, low sexual drive, and decreased sperm count.

Q2. What happens if the FSH level is high in males?

Ans. Increased level of FSH in males results in infertility. One such disorder is Klinefelter syndrome where a male child is born with an extra copy of the X chromosome.

Q3. What do you mean by negative feedback mechanism?

Ans. It is a self-regulatory mechanism in which the increased concentration of one hormone inhibits the activity of another hormone. For example, a rise in testosterone levels will inhibit the production of LH and FSH.

Q4. What is the clinical significance of LH?

Ans. Menotropin injections are used to treat infertility in women. These injections are made of FSH and LH which increase the production of these hormones in body.