Luteinizing Hormone (LH)

1. Overview

Luteinizing Hormone (LH) is a glycoprotein hormone that plays a crucial role in regulating reproductive function in both males and females. It is one of the two gonadotropins, along with follicle-stimulating hormone (FSH), that are essential components of the hypothalamic-pituitary-gonadal (HPG) axis. LH is responsible for triggering ovulation in females and stimulating testosterone production in males. Its secretion is governed by the pulsatile release of gonadotropin-releasing hormone (GnRH) from the hypothalamus.

2. Location

LH is synthesized and secreted by the gonadotroph cells of the anterior pituitary gland (adenohypophysis). Its release is stimulated by GnRH, which is secreted by the hypothalamus. Once released into circulation, LH acts on:

• Ovaries in females – particularly the theca cells of developing follicles and the corpus luteum.

• Testes in males – specifically the Leydig cells located in the interstitial tissue of the testes.

3. Structure

LH is a heterodimeric glycoprotein composed of two non-covalently bound subunits:

• Alpha (α) subunit: Common to other pituitary glycoproteins (FSH, TSH, and hCG).

• Beta (β) subunit: Unique to LH and determines its specific biological function.

The hormone is heavily glycosylated, which affects its half-life and activity. In humans, the molecular weight of LH is approximately 28–30 kDa.

4. Function

LH has distinct roles in male and female reproductive physiology:

• In females:

  • Triggers ovulation by causing the rupture of the mature Graafian follicle.

  • Stimulates the formation and maintenance of the corpus luteum.

  • Promotes secretion of progesterone and, to a lesser extent, estrogen from the corpus luteum.

• In males:

  • Stimulates Leydig cells in the testes to produce testosterone, which is essential for spermatogenesis and secondary sexual characteristics.

5. Physiological role(s)

LH plays a critical role in sexual development, fertility, and endocrine balance:

• Puberty: Rising LH levels contribute to sexual maturation by increasing gonadal steroid production.

• Menstrual cycle regulation: The mid-cycle LH surge is responsible for ovulation. Without this surge, the oocyte would not be released.

• Luteal phase support: LH sustains the corpus luteum, enabling adequate progesterone production to support potential implantation and early pregnancy.

• Spermatogenesis: In males, LH indirectly supports sperm production by stimulating testosterone, which acts on Sertoli cells in the seminiferous tubules.

• Feedback control: LH secretion is regulated by feedback from sex steroids (estrogen, progesterone, and testosterone) acting on the hypothalamus and pituitary.

6. Clinical Significance

Abnormal LH levels are associated with various reproductive and endocrine disorders:

• Elevated LH:

  • Polycystic Ovary Syndrome (PCOS): LH/FSH ratio is often elevated, contributing to anovulation and hyperandrogenism.

  • Primary gonadal failure (e.g., Turner syndrome, Klinefelter syndrome): LH is elevated due to loss of negative feedback from the gonads.

  • Menopause: Marked by high LH and FSH levels due to ovarian senescence and estrogen deficiency.

• Decreased LH:

  • Hypogonadotropic hypogonadism: Low LH due to hypothalamic or pituitary dysfunction results in decreased sex steroid production. Causes include pituitary adenomas, Kallmann syndrome, and functional hypothalamic amenorrhea.

  • Hyperprolactinemia: High prolactin levels suppress GnRH, thereby reducing LH and FSH secretion.

• Fertility assessment and treatment:

  • LH levels are used in evaluating ovulatory function and infertility in both sexes.

  • Ovulation predictor kits detect the LH surge to help time intercourse or insemination.

  • LH or hCG injections are used in assisted reproductive technologies to induce ovulation.

• LH and hCG overlap:

  • Human chorionic gonadotropin (hCG), produced during pregnancy, mimics LH and maintains corpus luteum function during early gestation.