Sensitivity, not abnormal hormones
Premenstrual syndrome and its more severe form, premenstrual dysphoric disorder, are not thought to stem from abnormal hormone levels. Studies generally find normal cyclic levels of estrogen and progesterone in affected individuals. The leading explanation is a heightened sensitivity of the brain to the normal hormonal shifts of the luteal phase, the stretch after ovulation and before menstruation.
Symptoms tend to appear in the luteal phase and ease soon after bleeding starts, a timing pattern that distinguishes these conditions from mood disorders that persist throughout the cycle.
The serotonin and GABA connection
A progesterone metabolite called allopregnanolone acts on GABA-A receptors, the brain's main inhibitory system, and its fluctuation is implicated in premenstrual mood and anxiety symptoms. The serotonin system is also involved, which helps explain why SSRIs can reduce PMDD symptoms, sometimes when taken only in the luteal phase.
Severity ranges widely, and PMDD in particular can be disabling. This is general educational information, not medical advice; persistent or severe premenstrual symptoms should be evaluated by a qualified clinician.