Luteinizing hormone

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Luteinizing hormone (LH) is a hormone synthesised and secreted by gonadotropes in the anterior lobe of the pituitary gland. It is one of the gonadotropins, which are necessary for proper sexual function, the other being follicle stimulating hormone (FSH).

Structure

LH is a glycoprotein, with a dimeric protein subunit, each monomer of which is connected to a sugar moiety. Its structure is similar to FSH, TSH, and hCG. The protein dimer contains 2 polypeptide units, labelled alpha and beta subunits. The alpha subunits of LH, FSH, TSH, and hCG are identical, containing 92 amino acids. Their beta subunits vary. LH has a beta subunit of 121 amino acids that confers its specific biologic action and is responsible for interaction with the LH-receptor.

The sugar part of the hormone is composed of fructose, galactose, mannose, galactosamine, glucosamine, and sialic acid, the latter being critical for its biologic half-life. The half-life of LH is only 20 minutes.

Genes

The gene for the alpha subunit is located on chromosome 6p21.1-23. It is expressed in different cell types. The gene for the LH beta subunit is located on chromosome 19q13.3, is expressed in gonadotropes of the pituitary cells and controlled by GnRH.

Activity

In both males and females, LH stimulates the production of sex steroids from the gonads. Leydig cells in male testes respond to LH by producing testosterone, while theca cells in the female ovary respond to LH by producing androgens and oestrogens.

In females, a LH surge about halfway through the menstrual cycle triggers the onset of ovulation. LH also induces the ovulated follicle to become the corpus luteum, which then secretes progesterone.

LH levels are normally low during childhood and, in women, high after menopause.

Deficient LH activity

1. Kallmann syndrome
2. Hypothalamic suppression
3. Hypopituitarism
4. Hyperprolactinemia
5. Gonadotropin deficiency
6. Gonadal suppression therapy
   1. GnRH antagonist
   2. GnRH agonist (downregulation)

Excess LH activity

1. Pituitary tumor (gonadotropes)
2. Polycystic ovary syndrome
3. Gonadal failure

Availability

LH is available mixed with FSH in the form of Pergonal and newer preparations of urinary gonadotropins, and pure in the form of recombinant LH.