Women's hormones
PMDD or something else? The blood tests worth doing first
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed October 20265 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
QUICK ANSWER
No blood test diagnoses PMDD. It is diagnosed from a symptom diary kept over at least two cycles. What a blood test can do is rule out look-alikes. An underactive thyroid, low iron stores or low vitamin B12 can all cause low mood and exhaustion, and can feel worse before a period.
Not sure which markers you need? Find my test →For a week or so every month, you are not yourself. You snap at people you love. You cry at nothing, sleep badly and feel flattened. Then your period starts and the fog lifts.
That pattern has a name, and you may have found it already. As of October 2026, threads asking what helps in the luteal phase, the two weeks between ovulation and your period, are climbing on Reddit’s supplement forums. Before you treat it as PMDD, it is worth knowing what else can look the same.
1. What is PMDD, and how is it diagnosed?
PMDD stands for premenstrual dysphoric disorder. The NHS describes it as a more severe form of PMS. The symptoms are similar but much more intense: anxiety, anger, deep low mood, poor sleep, headaches and aches.
“Between 2 and 4 in 100 women get PMS that is severe enough to prevent them from getting on with their daily lives.”
Royal College of Obstetricians and Gynaecologists, Managing premenstrual syndrome
The diagnosis rests on timing, not on a number. You track your symptoms day by day for at least two cycles in a row. PMDD means the symptoms sit in the days before your period and ease once it starts.
That diary matters more than it sounds. A 2024 review of 44 studies and 50,659 people in the Journal of Affective Disorders found PMDD in 3.2% when the diary confirmed it. Without the diary, the figure rose to 7.7%. Roughly half of the guesses do not hold up.
2. Which conditions can feel like PMDD?
Several common problems cause the same low mood and exhaustion. They tend to sit there all month, and a period can make them feel worse. A diary shows the shape. A blood test can check the cause.
| What it can feel like | Markers worth reading |
|---|---|
| Low mood, tiredness, heavy periods | TSH and free T4 (thyroid) |
| Wiped out, breathless, heavy bleeding | Ferritin, iron, transferrin saturation |
| Foggy, anxious, low, pins and needles | Vitamin B12 and folate |
| Flat and tired through the winter | Vitamin D |
TSH is the signal your brain sends to your thyroid, and free T4 is the hormone the thyroid makes back. The NHS lists low mood, exhaustion and heavy periods among the symptoms of an underactive thyroid. That is close to a PMDD checklist.
Ferritin is the protein that holds iron in reserve, so it shows how much iron you have banked. Heavy periods drain it month after month. Stores can run low while your haemoglobin still reads normal, which is why a full blood count alone can miss it.
The NHS lists mild depression and anxiety among the signs of low vitamin B12. And between October and early March, the NHS says we do not make enough vitamin D from sunlight, so low winter levels are common.
3. What can a blood test not tell you?
It cannot confirm PMDD. The NHS links PMDD to being very sensitive to changes in hormones. A sensitivity does not show up as a number, so one hormone result does not settle the question.
Progesterone and oestradiol are not part of any Helvy test. If a clinician wants those read, ask your GP. What we can measure is the list of look-alikes above, which is often the quicker win. If your cycle has also changed in your 40s, our perimenopause blood test guide covers that overlap.
4. What should you do next?
If you feel suicidal, call 999 or go to A&E now. The NHS is clear on this for PMDD. You can also call Samaritans free on 116 123, day or night.
- Start a daily symptom diary and keep it for two full cycles.
- Mark your period days, so the pattern shows at a glance.
- Note symptoms that never fully lift; those point away from PMDD.
- Take the diary and any blood results to your GP.
If a thread has you reaching for magnesium or vitamin D, measure first. Then you know your starting point, and whether anything changed.
A blood test measures biomarkers. It offers wellness insight rather than a diagnosis, and what any pattern means is a decision for a qualified clinician.
5. Which test checks the look-alikes?
The General Energy & Wellness panel (£149) covers most of the list. It measures TSH, free T4, vitamin D, vitamin B12, magnesium and cortisol, the stress hormone, alongside cholesterol and kidney markers. Cortisol is highest soon after waking, so note the time you take your sample.
If heavy periods are part of your story, the Vitamins & Minerals panel (£159) runs the full iron workup, including ferritin. It reads them with B12, folate, vitamin D and magnesium. Our heavy periods guide explains why ferritin matters there.
Not sure which fits? The Find my test tool walks you through it in a couple of minutes. For more on cycles and testing, see the women’s hormones hub.
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Frequently asked questions
Is there a blood test for PMDD?
No. PMDD is diagnosed from a daily symptom diary kept over at least two cycles in a row. Blood tests are used to rule out conditions that feel similar, such as an underactive thyroid, low iron stores or low vitamin B12.
How common is PMDD?
A 2024 review of 44 studies found PMDD in 3.2% of people when a symptom diary confirmed it, and 7.7% without one. The RCOG says 2 to 4 in 100 women get PMS severe enough to stop daily life.
Can a thyroid problem feel like PMDD?
It can. The NHS lists low mood, extreme tiredness and heavy periods among the symptoms of an underactive thyroid. A TSH and free T4 test reads your thyroid, and a qualified clinician can read the result alongside your diary.
Should I have my hormones tested for PMDD?
The NHS links PMDD to being very sensitive to changes in hormones, so one hormone result alone does not confirm it. Progesterone and oestradiol are not part of any Helvy test; ask your GP if a clinician wants them read.
When should I see a GP about PMDD symptoms?
See a GP if your symptoms affect daily life or lifestyle changes have not helped. Take two cycles of symptom diary with you. If you feel suicidal, call 999 or go to A&E straight away.
Related guides
Underactive Thyroid Symptoms
The signs that overlap with PMDD, and what TSH and free T4 show.
Low Ferritin, Normal Haemoglobin
How iron stores run low while a blood count still looks fine.
Low Mood Blood Test
The markers worth checking when your mood has dipped.
Anxiety Blood Test
Physical causes of anxiety that a blood test can read.
Heavy Periods Blood Test
What a normal count misses about your iron stores.
Women's Hormones Hub
Every guide we have written on cycles, hormones and testing.
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