Any problem that affects either the production, or coordinated action, of GnRH, FSH and LH will prevent ovulation.
PCOS is the commonest cause for anovulation accounting for around 80% of cases. In PCOS, the hormones GnRH, LH, FSH and oestrogen are being produced, and follicles containing eggs are present in the ovary, but there is lack of coordination amongst them. Due to this lack of coordination, a lead follicle does not develop to the advanced stage required for ovulation. Although women with PCOS often do not release eggs, ironically, their ovaries typically contain very high numbers of eggs.
Very high levels of the hormone, prolactin, can cause anovulation by blocking the production of GnRH. Like FSH and LH (which are called gonadotrophins), prolactin is produced by the pituitary gland. Prolactin is responsible for breast milk production in pregnancy. High levels of prolactin are often caused by a very small tumour in the pituitary (known as a microadenoma) but can also be caused by some medications such as antipsychotics and some antihypertensives. In other cases, the cause for high prolactin levels are unknown.
Stress and low body weight:
GnRH production can also become disrupted by having a very low bodyweight with low levels of body fat or by psychological stress. This is referred to as hypothalamic amenorrhoea.
Some individuals are unable to produce GnRH from birth. This condition is often associated with loss of the sense of smell and is known as Kallmann syndrome.
Lack of FSH and LH:
In rare cases, anovulation is due to an inborn lack of FSH and LH as in congenital hypopituitarism.
Loss of the ability to produce FSH and LH can also be due to a brain tumour.
Loss of FSH and LH may also happen after giving birth if the delivery was associated with very heavy blood loss. The resulting low blood flow starves the pituitary gland of oxygen causing it to become damaged. This condition is known as Sheehan’s syndrome.