The Hormonal Drivers of PMS

Introduction

One of the most confusing aspects of PMS is that laboratory tests usually come back normal. Women with PMS don\'t have abnormally high or low estrogen or progesterone — their hormone levels are, by most measures, within the expected range. So what causes PMS? The answer lies not in hormone levels per se, but in how the brain and nervous system respond to the normal hormonal fluctuations of the luteal phase.

The Hormonal Blueprint of the Luteal Phase

In the luteal phase (roughly Days 15–28

This hormonal rise and fall is the same in women with PMS and women without it. The critical difference appears to be in the brain\'s receptor sensitivity, neurotransmitter responsiveness, and neuroendocrine regulation.

Estrogen: The Serotonin Amplifier

Estrogen has profound effects on the serotonin system:

In the late luteal phase, falling estrogen reduces all of these serotonergic effects. For most women, this reduction is minor. For PMS-susceptible women, this fall may tip them into a state of relative serotonin deficiency — explaining the mood changes, irritability, anxiety, and carbohydrate cravings that characterise PMS.

This serotonin hypothesis is strongly supported by the remarkable effectiveness of SSRIs (serotonin reuptake inhibitors) in treating PMS/PMDD — even when used only in the luteal phase, not continuously. SSRIs can relieve PMDD symptoms within 1–2 days of administration, which is faster than their antidepressant effect — suggesting the mechanism is different from depression treatment.

Progesterone and Allopregnanolone: The Double-Edged Calmer

Progesterone itself does not directly cause most PMS symptoms. Rather, its metabolite — allopregnanolone (ALLO, also called 3α-tetrahydroprogesterone) — is the key actor. ALLO is a potent positive modulator of GABA-A receptors — the same target as alcohol and benzodiazepines. In most women, rising luteal ALLO produces mild sedation, reduced anxiety, and calming effects.

However, research from the Karolinska Institute and others has shown that in women with PMDD:

• ALLO paradoxically activates rather than inhibits excitatory neurons via a different GABA-A receptor subunit configuration (α4βδ subunits predominate)
• This produces anxiety
• irritability
• and mood instability — the opposite of the calming effect seen in other women
• The reason may be GABA-A receptor plasticity — the brain adapts to chronically high ALLO by reconfiguring receptors to a state that makes future ALLO exposure excitatory rather than inhibitory

This explains why some women feel worse with progesterone supplementation during the luteal phase — their nervous system responds to it adversely. It also explains why luteal-phase SSRI dosing works: by modulating serotonin (which interacts with GABA signalling

The HPA Axis and Cortisol

The hypothalamic-pituitary-adrenal (HPA) axis — the stress axis — is also dysregulated in PMS/PMDD. Several studies have found that women with PMDD show blunted or altered cortisol awakening response (CAR) compared to controls. The CAR is a sharp rise in cortisol within 30–45 minutes of waking, reflecting HPA axis responsiveness. A blunted CAR is associated with chronic stress, burnout, and mood disorders.

This HPA dysregulation may reflect an interaction between the ovarian hormones (particularly progesterone\'s cortisol-like structure, since both bind to the glucocorticoid receptor) and the stress system. Chronic stress can amplify PMS symptoms — and conversely, PMS itself is a chronic physiological stressor.

Why Some Women Are Susceptible and Others Are Not

The question of why some women are affected and others are not remains partly unanswered. Contributing factors include:

References: Bäckström T et al., Nat Rev Neurosci 2003; Hantsoo L, Epperson CN, Curr Psychiatry Rep 2015; ACOG Practice Bulletin 2023; Segebladh B et al., BJOG 2012.