Amenorrhea

Amenorrhea means no menstruation. It’s normally before puberty, during pregnancy and lactation and after menopause. Sometimes though menstruation either never starts which is called primary amenorrhea or suddenly stops in a person who's previously menstruating which is called secondary amenorrhea.

The menstrual cycle as a whole are controlled by the hypothalamus and the pituitary gland all the way up in the brain. The hypothalamus secretes gonadotropin releasing hormone or GnRH which makes the nearby anterior pituitary gland release follicle stimulating hormone or FSH and luteinizing hormone or LH in the first two weeks of an average 28-day cycle. The ovaries go through the follicular phase meaning that out of the many follicles scattered throughout the ovaries a couple of them enter a race to become the dominant follicle that will be released at ovulation. All the developing follicles secrete loads of oestrogen which negatively inhibits pituitary FSH. In the meantime the uterus goes through two phases the menstrual and the proliferative phase. During the menstrual phase the functional layer of the endometrium is shed and eliminated through the vagina leading to menstruation which lasts an average of five days. It's followed by the proliferative phase during which the rising levels of ovarian oestrogen makes the functional layer of the endometrium thicken and sprout endometrial glands additionally spiral arteries emerge to nourish the growing functional endometrium.  After ovulation the ovaries enter the luteal phase which lasts for the two weeks following ovulation. During the luteal phase the remnant of the ovarian follicle called the corpus luteum makes progesterone which negatively inhibits pituitary LH. Progesterone makes the endometrium go through the secretory phase during which it thickens some more and spiral arteries continue to grow if the egg is not fertilized by a sperm. Oestrogen and progesterone levels slowly decrease when progesterone reaches its lowest level. The spiral arteries collapse and the functional layer dies off and is eliminated through menstruation which marks the beginning of a new menstrual cycle.

Primary amenorrhea 

when a female hasn't had her first menstruation called menarche by age 16 despite normal growth and having started puberty. Primary amenorrhea is also suspected when a female hasn't had menarche by age 13 and doesn't show signs of puberty which include developing secondary sexual characteristics like breasts.
The most common cause of primary amenorrhea is Turner syndrome where one “X” chromosome is either completely or partially absent.
The most common karyotype is 45 X which means the person has 45 chromosomes of which only one is an “X” chromosome.
In Turner syndrome the ovaries are replaced by functionless fibrous tissue. This happens because the missing “X” chromosome leads to accelerated ovarian follicle depletion so that by two years old none are left essentially causing menopause before menarche no ovarian follicles also means no oestrogen and progesterone which leads to high levels of FSH and LH.

The second most common cause of primary amenorrhea is Mullerian Agenesis which is also called Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome in this case the mullerian duct system doesn't develop properly in female foetus. The mullerian duct system is responsible for the development of the uterus cervix and upper two-thirds of the vagina. So these organs may be absent or rudimentary and obstructed which explains the absence of menses but the ovaries develop normally in these individuals and the ovarian follicles make normal amounts of oestrogen and progesterone so there are normal levels of FSH and LH. 

A rare cause of primary amenorrhea is Androgen insensitivity syndrome. In this case the individual is biologically male which means they have a 46 XY karyotype but their androgen receptors don't respond to testosterone. So they don't have a uterus fallopian tubes or ovaries. Which explains the absence of menses but they have testicles which are usually in the abdomen or inguinal canal and they make up the normal amount of testosterone for a biologically male individual so FSH and LH levels are normal. Some of that testosterone gets converted into oestrogen so these people have female external genitalia and female secondary sex characteristics.

Endocrine disorders can also cause primary amenorrhea. These include Kallmann syndrome where a GnRH producing neurons fail to migrate from the nose region to the hypothalamus. During foetal development this causes low levels of GnRH, FSH and LH and as a consequence low oestrogen so puberty either never starts or is incomplete.

Secondary amenorrhea 

It defined as no menstrual bleeding for at least three normal menstrual cycles in a female who previously had regular cycles or for six months for females who used to have irregular cycles.

There are many causes of secondary amenorrhea and when they occur before menarche all of these can also cause primary amenorrhea.

The most common cause of secondary amenorrhea is pregnancy.

Hypothalamic amenorrhea which is when there's a decrease in GnRH secretion leading to low levels of LH, FSH and oestrogen. Often this is due to weight loss from anorexia nutritional deficiencies like excessively low fat consumption, prolonged periods of strenuous exercise or severe physical or emotional stress.

Another condition that affects the hypothalamic pituitary ovarian axis and can be responsible for secondary amenorrhea is polycystic ovary syndrome(PCOS).

It's not clear exactly what causes PCOS but there seems to be an imbalance between LH and FSH levels, specifically ovulation doesn't occurs so progesterone levels don't rise enough to inhibit LH production this leads to an increase in the production of LH compared to FSH. So there's an elevated LH to FSH ratio additionally because there's post ovulatory rise and fall in progesterone there's no menstruation issues with the pituitary gland can also cause secondary amenorrhea.

low levels of thyroid hormones can also cause amenorrhea. This is because low thyroid hormone levels induce the hypothalamus to release more thyrotrophic-releasing or TRH hormone until the pituitary to release more thyroid stimulating hormone or TSH.

When this happens TRH also stimulates prolactin release so amenorrhea occurs.

Sometimes secondary amenorrhea can occur because of premature ovarian failure which is when the ovarian follicles undergo accelerated atresia and get depleted before the age of 40 resulting in early menopause this leads to low serum oestrogen and high FSH and LH.