--- title: HPO axis: what it is and how it affects your cycle description: The HPO axis is the hormone feedback loop between your hypothalamus, pituitary gland and ovaries, and it shapes PMS and PMDD symptoms before your period. url: https://niyora.com/glossary/hpo-axis/ --- - [Niyora](https://niyora.com/) - [Glossary](https://niyora.com/glossary/) - HPO axis: what it means Glossary # HPO axis: what it means The HPO axis is the feedback loop between your hypothalamus, pituitary gland and ovaries that produces the hormones driving your menstrual cycle. It also interacts with your body's stress system, which is part of why mood can shift before your period. Researchers are still mapping exactly how this loop works in PMDD. Updated 14 October 2026 Niyora editorial Mixed evidence ## At a glance - The HPO axis is the feedback loop between your hypothalamus, pituitary gland and ovaries that drives ovulation and your cycle hormones. - Estrogen and progesterone from this loop rise and fall each month, and the luteal-phase drop is what triggers PMS and PMDD symptoms. - PMDD is linked to a different brain reaction to normal HPO axis hormone changes, not to abnormal hormone levels. - The body's stress system (the HPA axis) interacts with the HPO axis, which may shape how mood shifts across your cycle. - No single test checks your HPO axis for PMDD. Diagnosis relies on daily symptom tracking across at least two cycles. ## Strength of evidence Mixed evidence - None - Limited - Mixed - Moderate - Strong Meta-analyses link HPO axis hormones to mood and show lower luteal estradiol in PMDD, but cortisol differences across the cycle are small and often insignificant between finer phase comparisons, so exact mechanisms are still being mapped. ## What is the HPO axis? HPO stands for hypothalamic-pituitary-ovarian. It's the communication loop that drives your menstrual cycle, and it works in three steps: - Your hypothalamus releases a hormone called GnRH, in pulses. - Your pituitary gland responds by releasing FSH and LH. - Your ovaries respond to those signals by maturing an egg and making estrogen and progesterone. Each part listens to the others. When estrogen and progesterone rise or fall, your hypothalamus and pituitary adjust their signals in response, which is why doctors call this a feedback loop rather than a one-way switch. This loop is what triggers ovulation and sets the rhythm of the [menstrual cycle hormones](https://niyora.com/cycle/menstrual-cycle-hormones/) that shift across your month: - Before ovulation, rising estrogen prepares an egg for release. - After ovulation, a structure called the [corpus luteum](https://niyora.com/glossary/corpus-luteum/) takes over hormone production for about two weeks. - Once the corpus luteum breaks down, progesterone drops, which is usually when PMS and PMDD symptoms start. ## How does the HPO axis relate to PMS and PMDD? PMS and PMDD both show up in the second half of your cycle, the [luteal phase](https://niyora.com/luteal-phase/), when the HPO axis sends progesterone up, then down, before your period. That drop in estrogen and progesterone isn't unusual by itself. Everyone's hormones fall in the days before bleeding. What seems to differ is how the brain reacts to that normal drop. Schmidt et al. (1998, NEJM) and Dubey et al. (2017, Molecular Psychiatry) describe PMDD as a different brain reaction to typical HPO axis hormone changes, not a sign that your hormone levels are abnormal. One proposed piece of the picture is lower luteal estradiol in women with PMDD. Another is allopregnanolone, a hormone byproduct some brains react to more strongly ([what it means](https://niyora.com/glossary/allopregnanolone/)). This doesn't shrink what you're feeling. It means a normal biological process lands differently in your body than it does in someone else's. ## What does the research say about the HPO axis? Scientists are also looking at how the HPO axis interacts with the body's stress system, the HPA axis (hypothalamic-pituitary-adrenal). A few findings stand out: | Finding | What it shows | Source | | --- | --- | --- | | Cortisol shifts slightly by phase | Cortisol runs a little higher in the follicular phase than the luteal phase, across more than 2,600 women in 121 studies | [1] | | Lower luteal estradiol in PMDD | A meta-analysis of 5,129 women found lower luteal-phase estradiol in those with PMDD, compared with controls | [2] | | HPA axis shifts across reproductive life | The brain's stress-response system changes alongside hormone shifts at several reproductive stages, including before a period | [3] | | Shared pattern with postpartum depression | Hormone and immune-system changes may link PMS to mood changes after childbirth | [4] | This doesn't settle the question. One meta-analysis found slightly lower luteal estradiol in women with PMDD compared with controls, while the leading theory, from Schmidt et al. (1998) and Dubey et al. (2017), is that PMDD mainly involves a different brain reaction to largely normal hormone changes. ## Can you treat the HPO axis for PMDD? Guidelines don't target the HPO axis directly. They treat the pattern it produces. For PMDD, that usually means SSRIs (sometimes taken only in the luteal phase), some hormonal contraceptives, or CBT, and a doctor decides what fits your history. A few related terms worth knowing: - [Luteal phase](https://niyora.com/luteal-phase/): the second half of your cycle, when progesterone rises then falls. - [Corpus luteum](https://niyora.com/glossary/corpus-luteum/): the structure that makes progesterone after ovulation. - [Allopregnanolone](https://niyora.com/glossary/allopregnanolone/): a hormone byproduct some PMDD brains react to differently. - [PMS and PMDD research, summarised](https://niyora.com/research/): more studies on hormones, mood and the cycle. Niyora is a PMS and PMDD support app. Its Moon feature lets you write down what happened in a hard moment, see it from another angle, calm down and draft a reply you can hold onto before you send anything. When to see a doctor If this pattern takes a week of every month, or you have thoughts of hurting yourself, see a GP or gynecologist and bring two months of daily notes. ## Questions people ask What does HPO axis stand for? + HPO axis stands for hypothalamic-pituitary-ovarian axis, the feedback loop between your brain and ovaries. It releases the hormones that drive ovulation and your monthly cycle, including the estrogen and progesterone swings of the luteal phase. Is the HPO axis the same as the HPA axis? + No. The HPO axis controls hormones that drive ovulation, while the HPA axis (hypothalamic-pituitary-adrenal) controls your body's stress response. The two interact. Cortisol runs slightly higher in the follicular phase than the luteal phase (Frontiers in Neuroendocrinology, 2022), and stress hormones may shape how PMDD symptoms feel. Does the HPO axis cause PMDD? + PMDD seems to involve a different brain reaction to largely normal hormone changes rather than abnormal hormones overall, though one meta-analysis (Frontiers in Neuroendocrinology, 2021) found lower luteal estradiol in women with PMDD compared with controls. Can you test or fix your HPO axis? + There's no simple blood test that diagnoses PMDD through the HPO axis. Guidelines recommend tracking symptoms daily for two cycles, and treatments like SSRIs, some hormonal contraceptives or CBT target the pattern, not a single hormone number. Why does the HPO axis affect mood before my period? + The HPO axis makes progesterone and estrogen fall before your period. Those hormones affect brain chemicals tied to mood, and some brains, including those with PMDD, react more strongly to that normal drop. ## Sources - 1. [HPA axis activity across the menstrual cycle - a systematic review and meta-analysis of longitudinal studies](https://doi.org/10.1016/j.yfrne.2022.100998), Frontiers in neuroendocrinology, 2022 - 2. [Variation in genes and hormones of the hypothalamic-pituitary-ovarian axis in female mood disorders - A systematic review and meta-analysis](https://doi.org/10.1016/j.yfrne.2021.100929), Frontiers in neuroendocrinology, 2021 - 3. [The role of the hypothalamic-pituitary-adrenal axis in depression across the female reproductive lifecycle: current knowledge and future directions](https://doi.org/10.3389/fendo.2023.1295261), Frontiers in endocrinology, 2023 - 4. [The cytokine-hormone axis - the link between premenstrual syndrome and postpartum depression](https://doi.org/10.1080/09513590.2017.1318367), Gynecological endocrinology : the official journal of the International Society of Gynecological Endocrinology, 2017 Ask AI about this page Copy prompt ChatGPT ↗ Claude ↗ Perplexity ↗ Google AI Mode ↗ Grok ↗ On this page - [What is the HPO axis?](#what-is-the-hpo-axis) - [How does the HPO axis relate to PMS and PMDD?](#how-does-the-hpo-axis-relate-to-pms-and-pmdd) - [What does the research say about the HPO axis?](#what-does-the-research-say-about-the-hpo-axis) - [Can you treat the HPO axis for PMDD?](#can-you-treat-the-hpo-axis-for-pmdd) - [Questions](#faq) Niyora PMS & PMDD support For the hard moment before your period. Private, no account. [Get it on iPhone](https://apps.apple.com/app/niyora-pms-emotions/id6764561688) ## Keep reading Condition PMDD (premenstrual dysphoric disorder), explained Condition The luteal phase: what's happening in your body and mood Question What causes PMDD? Cycle guide Menstrual cycle hormones, phase by phase Guide All 26 research findings Term Corpus luteum: what it means Term What Does PMS Mean in Text and Slang? Term Allopregnanolone: what it means for PMDD