The heaviness that settles in the week before your period. The irritability that seems to come from nowhere. The low mood that lifts almost the moment your period arrives. It is easy to write these off as ordinary premenstrual feelings. But if your mood drops in a way that tracks your cycle and disrupts your daily life, that pattern is worth taking seriously. Understanding what might be behind persistent low mood across your cycle is the first step towards finding the right support.
The difference between PMS and PMDD
PMS, or premenstrual syndrome, is common, and it comes from the natural hormone shifts that happen before your period. The symptoms tend to be milder: mood swings, feeling irritable or tearful, tiredness, and bloating. They usually ease once your period starts.5
Premenstrual Dysphoric Disorder (PMDD) is a more severe condition. It is a hormone-based mood disorder, where symptoms build during the luteal phase, the one to two weeks before your period, and settle within a few days of your period arriving. PMDD comes from a heightened sensitivity to the normal rise and fall of oestrogen and progesterone across your cycle. Your hormone levels themselves are usually normal.6
With PMDD, the symptoms are severe enough to disrupt your daily life. On the mood side, you might feel deep depression, hopelessness, anxiety, panic attacks, intense anger or irritability, and sharp mood swings. Physically, you may notice fatigue, bloating, tender breasts, and headaches. Suicidal feelings can also be part of PMDD. If you experience these, support is available in the callout below.7
When your thyroid might be playing a part
If your low mood does not follow a cyclical pattern, or it comes alongside physical symptoms like fatigue, cold sensitivity, or weight changes, it is worth discussing your thyroid with a GP. An underactive thyroid (hypothyroidism) can cause low mood, depression, anxiety, mood swings, and difficulty concentrating. Because these symptoms overlap with many mental health conditions, a simple blood test is often used to check your hormone levels.19
An underactive thyroid can also change your cycle itself. Your periods might get heavier, longer, or more irregular, and in some cases they stop altogether. Alongside this, you may notice fatigue, weight gain, and feeling the cold more than usual.2
If you go to a GP with low mood or depression, they can do a blood test to help rule out an underactive thyroid. It is a straightforward step, and it can clear up the picture a lot.10
How everyday habits feed into your mood
Everyday lifestyle habits have a real impact on mental wellbeing, and their effects can feel especially pronounced during the premenstrual phase when you are already more vulnerable to mood shifts. Eating regular, balanced meals helps keep your blood sugar steady. When blood sugar drops or swings sharply, it can trigger sudden crashes in energy, increased irritability, and lower mood. Poor sleep tends to pull your mood down, raise anxiety, and make it harder to concentrate. Moving your body triggers the release of endorphins, feel-good brain chemicals that naturally lift your mood, relieve physical tension, and take the edge off stress.3411
Ongoing stress can feed into your low mood and anxiety. If you feel cut off from others, that can make mental health struggles more likely, while staying connected tends to offer some protection. Spending time outdoors may also take the edge off your stress and anger.12
Spotting the pattern, and why tracking helps
The way to pin down PMDD is to track your symptoms every day across at least two cycles in a row. Blood tests and hormone tests cannot confirm it. Tracking shows whether your symptoms are genuinely cyclical, turning up in the week or two before your period and easing once it starts. Without that record, cyclical mood changes are hard to tell apart from general low mood or something else.13
You can track your symptoms with a paper diary or an app. Noting both your mood and physical symptoms each day builds a clear picture to bring to a GP. A record like this helps a GP tell hormonal mood shifts apart from other emotional changes, and it makes for a more useful conversation about what comes next.14
When to seek support and what to expect
It is worth seeing a GP if low mood or depression sits with you for most of the day, every day, for more than two weeks. The same goes if it is affecting your daily life, work, or relationships, or if changes to your habits have not helped. If you are worried at any point before that two-week mark, reach out sooner rather than waiting.15
When you book a GP appointment, bringing your symptom diary along can help. Your GP can ask about what you have been going through, may arrange a blood test to rule out something like an underactive thyroid, and can talk through support options, which might include therapy or medication.16