Introduction

The Gonadotropic hormones or the Gonadotropins are a group of hormones that are released from the anterior pituitary gland in vertebrates. It includes two primary hormones namely, Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) that act on the gonads (the ovaries and testes) of the mammals and help in sexual development and the process of reproduction.

Apart from these two anterior pituitary hormones, the gonadotropins also include human chorionic gonadotropin (HCG) that is released during the time of pregnancy from the placenta. The hormone family also includes equine chorionic gonadotropin (released in female horses during pregnancy) and two of fish gonadotropins as well.

Gonadotropic cells

The gonadotropic cells also known as gonadotrophs are the cells that secrete the gonadotropic hormones. The gonadotrophs are endocrine cells situated in the anterior pituitary gland and are stimulated by the gonadotropin releasing hormones (GnRH).

The gonadotrophs have insulin receptors on their surface; high levels of insulin can disrupt the hormone releasing levels and lead to infertility. The cells are also inhibited by the hormone estradiol.

Now that we know about the gonadotropic hormones and the cells that release them. Let us study their primary hormones in detail below.

Luteinizing Hormone

Luteinizing hormone (LH) or interstitial cell stimulating hormone (ICSH) is one the gonadotropic hormones that is released by gonadotropic cells of the anterior pituitary gland. It is a heterodimeric glycoprotein hormone that has one alpha and one beta subunit to make it a fully functional protein.

Structure

The alpha and the beta subunits are connected by a non covalent bond. The alpha subunit is made up of 92 amino acids in humans and upto 96 amino acids in other vertebrates. These hormones are not found in invertebrates.

The beta subunit is made up of upto 120 amino acids and constitutes a specific sequence that stimulates the LH receptor. The LH works in conjunction with FSH to stimulate various sexual developments in both males and females. Let us look at those functions now.

LH in Males

The luteinizing hormones stimulate the release of androgens in both males and females. In males, they help in the production of testosterone from the Leydig cells under the regulation of gonadotropin releasing hormone.

The luteinizing hormones bind to the LH receptor on the surface of leydig cells, increasing the levels of cAMP molecules. This increase leads to the translocation of cholesterol into mitochondria where it is first converted into dehydroepiandrosterone (DHEA) and then finally into testosterone.

The secretion of the hormone testosterone is pulsatile in nature. The LH levels are low at the time of puberty but increase with great surge at the time of puberty.

LH in females

The luteinizing hormones bind to the receptors present on the ovarian follicles and promote their growth. During the menstrual cycle, there is a surge in the secretion of the LH hormones which is important for the rupture of the Graffian follicle. LH also stimulates the secretion of progesterone in females which helps in transforming the Graffian follicle into corpus luteum and thus completing the cycle of ovulation. LH is important in females as it helps in the secretion of endometrium walls in the uterus and also the implantation of fertilised eggs.

Trends of the LH

A surge in LH levels in the second week of menstrual cycle in females causes the release of egg from the ovary, indicating high chances of getting pregnant. Thus, in females with regular menstrual cycles the LH increases every month to complete the process of ovulation and also supports the secretion of progesterone. In older females who are menopausal, the levels of LH are high but the levels of progestrone and estrogen are low.

The LH levels in males rises after puberty for sexual growth and development. It maintains a normal level throughout the male life after puberty.

There are very low levels of LH in infants and children. It rises only after puberty.

Follicle Stimulating Hormone

Follicle stimulating hormone (FSH) is another gonadotropic hormone released by the anterior pituitary gland. It is required for the development and functioning of reproductive organs in both males and females. It is named so because of its ability to stimulate follicle development in females.

Structure

It is a heterodimeric glycoprotein that is composed of an alpha and a beta subunit. The FSH alpha subunit is similar to the LH alpha subunit. The beta subunit of FSH is composed of 111 amino acids with a specific sequence that interacts with the follicle stimulating hormone receptor (FSHR).

FSH in Males

In males, follicle stimulating hormone is responsible for the formation of gametes. It stimulates the division of primary spermatocytes to form secondary spermatocytes. It also induces the Sertoli cells (site of spermatogenesis) to release androgen binding proteins that help in the process of sperm production.

FSH in females

In females, the follicle stimulating hormone helps in the maturation of the ovarian follicles. The hormone binds to the receptors on ovarian follicles and acts as a major factor for their growth. The FSH surges in between the menstrual cycle and promotes ovulation. It also stimulates the production of oestrogen that helps in the maturation of the Graffian follicle.

Similar to LH, the FSH surges in a cyclical manner in females to complete the process of ovulation.

Trends of the FSH

In men, the FSH levels stay constant after a sudden rise during puberty. This constant level is important for the development of spermatozoa.

In women, the FSH level fluctuates according to their menstrual cycles. The level is the highest after the release of egg. In perimenopausal (transition from menstruation to menopause) women, the FSH is high one day and lower the next day. In other words, there is no fixed pattern for FSH levels in those women.

Inhibin secreted by the Sertoli cells and Graffian follicle negatively inhibit the secretion of FSH.

Human Chorionic Gonadotropin

The human chorionic gonadotropin (hCG) is one of the earliest hormones produced by the trophoblast cells of the embryo. This hormone is used as a parameter to detect pregnancy in early stages.

Structure

Similar to LH and FSH, it is a heterodimeric glycoprotein that is composed of an alpha and a beta subunit. The alpha subunit is similar to the LH and FSH subunit and is made up of 92 amino acids.

The beta subunit is composed of 145 amino acids. The two subunits together create a hydrophobic core but the outer amino acids are mostly hydrophilic.

Function

• The hCG hormone binds with the LH receptor and helps in the maintenance of corpus luteum.
• It is the parameter for recognition of maternal pregnancy.
• The hCG hormone also acts as a link for the development of immune tolerance.
• Some evidence also suggests that the presence of hCG hormones leads to morning sickness in women during the time of pregnancy.

Forms of hCG

The hCG hormone exists in three forms – regular hCG, hyperglycosylated hCG and free beta subunit of hCG. The regular and beta hCG levels are used for the detection of pregnancy. The hyperglycosylated hCG is mainly secreted during the time of implantation.

Stimulation of the Gonadotropic Hormones

The gonadotropic hormones – follicle stimulating hormone and luteinizing hormone are stimulated by another hormone called Gonadotropin releasing hormones, abbreviated as GnRH. The GnRH is secreted in the hypothalamus by gonadotropin neurons. It belongs to the family of gonadotropin-releasing hormone family.

Structurally, it is a decapeptide which can be identified by the following amino acid sequence –

pyroGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH₂

The GnRH is released in a pulsatile manner from the hypothalamus and reaches the pituitary gland via the portal bloodstream. Upon reaching the pituitary gland, the hormone activates its own receptor that is present on the surface of gonadotropic cells. This leads to the release of calcium and protein kinase C which promotes secretion of LH and FSH.

The activity of GnRH is low during birth and in children. It increases once the child reaches puberty. At puberty, the pulsatile release of GnRH stimulates the production of LH and FSH. This mechanism is negatively inhibited by androgens and estrogens. Low pulses of GnRH are required for FSH secretion, whereas high pulses of GnRH are required for LH secretion.

In males, the GnRH is released in pulses at a constant frequency. However in females the GnRH levels fluctuate according to the menstrual cycle. It is at the highest level just before ovulation.

Related Diseases

1. High Levels of FSH

Higher levels of FSH is usually seen in women with menopause that accompanies hot flashes and vaginal itchiness and dryness. High levels of FSH during reproductive years can be indicative of –

• Absence of normal inhibition mechanism
• Premature menopause
• Klinefelter syndrome, Turner syndrome or Gonadal dysgenesis
• Castration
• Lupus
2. Low Levels of FSH

Diminished secretion of FSH can cause gonadal failure, known as hypogonadism. In males, there is no or decreased production of sperm whereas in females normal menstrual cycle is hindered. Low levels of FSH is also seen in other conditions such as –

• Polycystic Ovarian Syndrome
• Hypothalamic suppression
• Gonadotropin deficiency
• Kallmann syndrome
3. High Levels of LH

The levels of LH is normally high in menopausal women, but if it is abnormally high in the reproductive years, it can be indicative of –

• Congenital adrenal hyperplasia
• Klinefelter syndrome, Turner syndrome or Gonadal dysgenesis
• Testicular failure
• Polycystic Ovarian Syndrome

Note: Children who undergo puberty at a rather early age show high levels of LH and FSH according to their age.

4. Low Levels of LH

Similar to FSH, low levels of LH can cause hypogonadism which causes failure in production of sperm in males and irregular menstruation (amenorrhea) in females. Other conditions that show low levels of FSH are –

• Eating disorder
• Pasqualini syndrome
• Kallmann syndrome
• Female athlete triad
5. Others
• GnRH insensitivity: It is caused by a rare autosomal mutation that leads to inactivity of the gonadotropin releasing hormone receptor and thus sex hormones are not synthesised.
• LH Insensitivity: Also known as Leydig cell hypoplasia is a rare genetic autosomal disorder that occurs in males. It renders the body unresponsive to LH leading to infertility.
• FSH Insensitivity: Also known as ovarian follicle hypoplasia is a rare autosomal genetic disorder in females that makes the body insensitive to FSH and causes amenorrhea, reduced puberty or total infertility.

This sums up the detailed overview of the gonadotropic hormones. Stay tuned to BYJU’S for more important information related to NEET.

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