Vitamin D behaves less like a vitamin and more like a hormone. It is made in your skin from sunlight, converted by your liver and kidneys, then binds to receptors found in your ovaries, uterus lining, pituitary gland and pancreas, which is why low levels have been linked in research to insulin resistance in PCOS (now also known as PMOS), lower fertility markers, more severe PMS, and a rougher perimenopause. Around 1 in 6 adults in the UK have low vitamin D status, and the NHS recommends everyone consider a daily 10 microgram (400 IU) supplement, particularly from October to March when UK daylight is too weak to make it in skin. Below is what the research actually shows about vitamin D and women's hormonal health, at every life stage, plus how much you actually need and when to get tested.

What Does Vitamin D Actually Do for Your Hormones?

Unlike most vitamins, which come only from food, vitamin D is synthesised in your skin when it's exposed to UVB sunlight, then converted in the liver to 25-hydroxyvitamin D (the form measured in a blood test) and finally activated in the kidneys into calcitriol, the hormone that actually does the work. Calcitriol then travels through the bloodstream and binds to vitamin D receptors, which researchers have found in the ovaries, the lining of the uterus, the pituitary gland, the pancreas and fat tissue.

That distribution matters for hormonal health specifically. Vitamin D receptors sit on the cells involved in producing oestrogen and progesterone (steroidogenesis), on the beta cells of the pancreas that manage insulin, and on immune cells that influence inflammation. This is the biological reason vitamin D keeps coming up in research on PCOS, fertility, PMS and menopause: it isn't a coincidence, it's a nutrient sitting at a genuine crossroads of hormone production, blood sugar regulation and mood.

How Common Is Vitamin D Deficiency Among UK Women?

Public Health England data suggests roughly 1 in 6 UK adults have low vitamin D status, and the proportion is higher in winter, when the sun sits too low in the sky for the skin to make any vitamin D at all between approximately October and March. A blood level under 25 nmol/L is generally classed as deficient, with 25 to 50 nmol/L considered insufficient for some people, though your GP is the right person to interpret your specific result.

Who's at higher risk	Why it happens
Living in the UK, October to March	The sun's angle is too low for skin to produce vitamin D, whatever your time outdoors
Mostly indoor or office-based days	Less skin exposed to daylight overall, year-round
Darker skin tones	More melanin means the skin needs more sun exposure to make the same amount of vitamin D
Pregnant or breastfeeding	Nutrient demand rises to support the baby's bone development as well as your own
Higher body weight	Vitamin D is fat-soluble and can be held in fat tissue, lowering the amount circulating in blood
Managing PCOS/PMOS	Several studies find women with PCOS test low more often than women without it, independent of weight

Can Vitamin D Affect PCOS and Insulin Resistance?

This is one of the most researched corners of vitamin D science in women's health. Multiple systematic reviews and meta-analyses have found that women with PCOS are, on average, more likely to have low vitamin D levels than women without it, and that lower levels tend to track alongside markers of insulin resistance such as HOMA-IR. Some randomised trials of vitamin D supplementation in women with confirmed deficiency have shown modest improvements in insulin sensitivity and inflammatory markers, though the research is mixed and not every trial finds a benefit, and correlation in observational studies does not prove that low vitamin D causes insulin resistance rather than simply travelling alongside it.

The sensible, evidence-consistent takeaway is not that vitamin D treats or reverses PCOS (it doesn't, and no supplement legally can claim that), but that correcting a genuine deficiency is a reasonable, low-risk piece of the wider picture alongside diet, movement and any other supplements or medication your GP has recommended.

Does Vitamin D Make a Difference to Fertility?

A widely cited systematic review and meta-analysis published in Human Reproduction found that women with sufficient vitamin D levels going into assisted reproduction (IVF/ART) had higher rates of clinical pregnancy and live birth than women with insufficient levels. Vitamin D receptors are present in the endometrium and in developing follicles, which gives a plausible biological mechanism, and some fertility clinics now check vitamin D status as a routine part of pre-conception workups.

It's worth being precise about what this evidence does and doesn't show: these are largely observational associations rather than proof that supplementing vitamin D on its own improves the odds of conceiving. But because deficiency is common, cheap and straightforward to test and correct, and because folate, iron and vitamin D are already part of standard pre-conception advice, checking your level (or covering the gap with a well-formulated supplement) is a sensible, low-risk step if you're trying to conceive.

Can Vitamin D Help With PMS, Mood and Perimenopause?

A randomised trial in vitamin D deficient students found that correcting the deficiency reduced several inflammatory and antioxidant markers linked to PMS severity, and observational research has repeatedly linked low vitamin D with worse premenstrual mood symptoms and, in some studies, more severe PMDD. Vitamin D is thought to play a role in the pathways that regulate serotonin, which may explain the mood connection, though as with PCOS, the evidence is stronger for association than for guaranteed cause and effect.

During perimenopause and beyond, vitamin D's best-established job is supporting bone health, working alongside calcium as oestrogen (which normally helps protect bone density) starts to decline. Some research also explores a wider role for vitamin D in mood and cognitive function through midlife, though this evidence base is earlier stage than the bone data.

How Much Vitamin D Do You Actually Need, and When?

The NHS recommends that everyone in the UK consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D, particularly between October and March, since UK sunlight isn't strong enough to produce it in skin during those months regardless of how much time you spend outside. Some groups are advised to take this amount all year round.

Group	NHS guidance	Why
General adult, spring/summer	Consider 10mcg (400 IU) daily	Skin can often make enough from April to September
General adult, October to March	10mcg (400 IU) daily	UK sunlight is too weak to produce vitamin D in skin
Pregnant or breastfeeding	10mcg (400 IU) daily, discuss with your GP or midwife	Supports both maternal and baby's bone development
Darker skin, always covered skin, over 65, or rarely outdoors	10mcg (400 IU) daily, all year round	Reduced skin production of vitamin D from sunlight
Confirmed deficiency on a blood test	Higher, time-limited dose as prescribed by your GP	Safely corrects a confirmed low level under supervision

Vitamin D is fat-soluble, which means it's stored in the body rather than passed out quickly, so it's possible to take too much. The NHS advises not to exceed 100mcg (4000 IU) a day from all sources unless a doctor tells you to, and symptoms of taking too much are rare but can include nausea and kidney problems over time.

FAQs: Vitamin D and Women's Hormonal Health

Can I get enough vitamin D from food alone?

It's difficult in the UK. Oily fish, egg yolks, red meat and fortified foods like some cereals and plant milks contain vitamin D, but the amounts are small relative to what most people need, especially in winter, which is why the NHS recommends supplementing rather than relying on diet alone.

What's the difference between vitamin D2 and D3?

D3 (cholecalciferol) is generally considered more effective at raising and maintaining blood vitamin D levels than D2 (ergocalciferol). MyOva's supplements use a vegan D3 sourced from lichen rather than the traditional animal-derived (lanolin) form.

Should I get my vitamin D level tested before supplementing?

A standard 10mcg (400 IU) daily dose is considered safe for most adults without testing first. But if you have symptoms like ongoing fatigue, bone or muscle aches, or you're trying to conceive and want a fuller picture, ask your GP about a blood test so you know your actual starting point.

Does vitamin D interact with any medication?

It can interact with certain medications, including some used for epilepsy, steroids, and drugs that affect calcium levels. Always check with your GP or pharmacist before starting a new supplement if you take regular medication.

Is vitamin D a treatment for PCOS, PMDD or infertility?

No. Vitamin D is a food supplement, not a licensed medicine, and it does not diagnose, treat, cure or prevent any medical condition. Correcting a genuine deficiency may support your overall hormonal and metabolic wellbeing as part of a wider picture that includes diet, lifestyle and any care your GP has recommended.

MyOva products are food supplements, not medicines, and are not intended to diagnose, treat, cure or prevent any disease. Always consult your GP or pharmacist before starting a new supplement, especially if you are pregnant, breastfeeding, taking medication, or managing a diagnosed medical condition.

Sources: NHS: Vitamin D; Pal et al., "Vitamin D and assisted reproductive treatment outcome: a systematic review and meta-analysis," Human Reproduction (2018); systematic reviews and meta-analyses on vitamin D, insulin resistance and PCOS, PMC/PubMed (2024-2025); "The effects of vitamin D on mood alteration in women's life: focus on depression," PubMed (2023); "Vitamin D Supplementation for Premenstrual Syndrome-Related inflammation and antioxidant markers," Scientific Reports (2019); "The role of vitamin D in menopausal women's health," Frontiers in Physiology (2023).

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