Vitamin D and your hormones: more than a bone vitamin

Vitamin D is the most-studied nutrient in women's health where the story is bigger than the textbook. Most of us learned that vitamin D is "for bones." That is true and incomplete. Vitamin D is actually a steroid hormone with receptors in nearly every tissue in the body, including the ovaries, the pituitary gland, the hypothalamus, the thyroid, and the immune system. Its deficiency affects substantially more than bone density.

For European women specifically (where vitamin D deficiency is common due to high latitudes and limited winter sun), the implications stretch into menstrual regularity, PMS severity, PCOS, thyroid autoimmunity, and fertility.

This article is the companion to our vitamin D3 ingredient page, covering the hormonal angle the ingredient page does not.

Vitamin D is not really a vitamin

Vitamin D is structurally a steroid hormone. The biologically active form (1,25-dihydroxyvitamin D, also called calcitriol) binds to the vitamin D receptor (VDR), a nuclear receptor that regulates the transcription of hundreds of genes.

VDRs are expressed in: - Bone and gut (the classical sites) - Ovaries, uterus, placenta (reproductive) - Hypothalamus and pituitary (HPO axis regulation) - Thyroid (autoimmune regulation) - Most immune cells (T cells, B cells, dendritic cells) - Skin, brain, muscle, fat, pancreas, and many other tissues

This explains why vitamin D deficiency produces such varied symptoms beyond bones. Vitamin D is more accurately described as a systemic hormone whose deficiency affects multiple organ systems.

Vitamin D and the menstrual cycle

A 2016 cross-sectional study of African-American women found that lower serum vitamin D was associated with higher odds of long menstrual cycles ^[1]. Subsequent studies have replicated the link between vitamin D deficiency and menstrual irregularity in different populations ^[2].

The mechanisms proposed:

• VDR expression in the hypothalamus suggests vitamin D affects GnRH release ^[3]
• Vitamin D regulates aromatase, the enzyme that converts androgens to estrogens ^[3]
• Vitamin D supports corpus luteum function and adequate progesterone production ^[3]
• Vitamin D modulates AMH (anti-Müllerian hormone), a marker of ovarian reserve

The clinical implication: women with irregular cycles and unexplained ovulation issues are worth checking for vitamin D status. In some studies, supplementation has restored menstrual regularity, particularly in women with significant deficiency.

Vitamin D and PMS

A 2018 study of women with musculoskeletal pain found that vitamin D deficiency, low calcium intake, and psychological symptoms were interrelated factors in PMS severity ^[4]. Multiple trials have suggested vitamin D supplementation can improve PMS symptoms, particularly mood-related symptoms, in deficient women.

The mechanism likely involves: - Vitamin D's effects on serotonin synthesis (it is a cofactor in tryptophan-to-serotonin conversion) - Vitamin D's regulation of estrogen and progesterone, the hormones whose late-luteal drop drives PMS (see our Hormones and mood article) - Anti-inflammatory effects that reduce the prostaglandin and cytokine load that amplifies PMS symptoms

The evidence is not strong enough to claim vitamin D treats PMS, but it is strong enough to suggest that women with significant PMS should ensure their vitamin D status is adequate.

Vitamin D and PCOS

Medical disclaimer. PCOS is a recognised medical condition that requires diagnosis by a qualified healthcare professional. Diagnosis follows the Rotterdam criteria, refined in the 2023 International Evidence-Based Guideline. Nutrition and supplementation can support women with PCOS but do not treat or cure it.

PCOS is one of the conditions where the vitamin D evidence is strongest:

• 84.1% of women with PCOS in one cross-sectional study had vitamin D deficiency ^[5]
• Multiple RCTs show vitamin D supplementation reduces total testosterone by ~12% and androgens by ~17% in women with PCOS ^[6]
• Vitamin D improves insulin sensitivity in PCOS, which is one of the central metabolic features of the condition
• A 12-week clinical trial of 60,000 IU vitamin D per week in women with PCOS showed improvements in menstrual regularity, acne, hirsutism, mood swings, and weight control, with biochemical normalisation of reproductive hormones (decreased testosterone, LH, AMH; increased FSH, estrogen, progesterone) and reduced insulin resistance (lower fasting insulin and HOMA-IR) - Combined vitamin D + omega-3 + myo-inositol has emerging evidence for PCOS management

For women with PCOS, vitamin D supplementation has stronger evidence than most other nutritional interventions and is part of multiple clinical guidelines.

Vitamin D and thyroid autoimmunity

Medical disclaimer. Hashimoto's thyroiditis is a recognised autoimmune condition that requires diagnosis by a qualified healthcare professional, typically through TSH, free T4 and thyroid antibody testing. Vitamin D supplementation may be supportive but does not replace levothyroxine or other medical treatment for diagnosed hypothyroidism.

Hashimoto's thyroiditis (the most common cause of hypothyroidism, predominantly affecting women) shows a notable association with vitamin D deficiency:

• Vitamin D deficiency is more common in women with Hashimoto's than in matched controls ^[7]
• Vitamin D supplementation in Hashimoto's patients has shown reductions in antithyroid antibody titers in some trials ^[8]
• The mechanism involves vitamin D's modulation of T-cell function and reduction of autoimmune attack on the thyroid

The evidence is not yet definitive enough for vitamin D to be a primary treatment for Hashimoto's. It is strong enough for testing and supplementation to be reasonable adjunctive practice for women with Hashimoto's, particularly those who are deficient.

The same logic applies to other autoimmune conditions more common in women (rheumatoid arthritis, lupus, multiple sclerosis): vitamin D status appears to affect both risk and severity.

The European deficiency picture

This is where the practical recommendation gets specific.

Vitamin D production requires UVB exposure (sunlight). In Northern European latitudes (above ~40°N, which includes most of Germany, Austria, Switzerland, UK, Netherlands, Scandinavia), the UVB angle is insufficient for vitamin D synthesis from approximately October to April. During winter, dietary sources and stored vitamin D are the only sources.

The German DEGS1 study found 69.9% of women aged 65 to 79 had low vitamin D levels ^[9]. Population studies across Europe show:

• Around 13% of European adults have severe vitamin D deficiency (<25 nmol/L or <10 ng/mL)
• Around 40% have inadequate levels (<50 nmol/L or <20 ng/mL)
• Higher rates in older women, women of color, women who cover for religious reasons, women with limited outdoor time, and women in the higher-latitude countries

For European women, particularly in winter, vitamin D supplementation is one of the better-evidenced supplement decisions.

How much to take

The EFSA reference intake for vitamin D is 15 μg/day (600 IU). The German DGE recommends 20 μg/day (800 IU) when sun-derived vitamin D is inadequate (i.e., October to April for most German women).

Therapeutic doses for deficiency treatment can be higher (typically 1,000 to 4,000 IU/day, sometimes 50,000 IU weekly under medical supervision). The safe upper limit (UL) for long-term supplementation is generally set at 4,000 IU/day for adults ^[10].

The most useful step: get your 25-hydroxyvitamin D level tested. The optimal range is generally considered 75 to 125 nmol/L (30 to 50 ng/mL). If you are below this, supplementing toward this range has the most defensible evidence. If you are well within this range, additional supplementation produces diminishing returns.

Form, dose, and what to pair with

Form: vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol) at raising and maintaining blood levels (see our Why the form matters article). D3 is what nōuxx uses.

Dose: 1,000 to 2,000 IU/day is reasonable maintenance for most adult women in Northern Europe in winter. Higher doses (3,000 to 4,000 IU/day) can be appropriate for treating documented deficiency under medical guidance. Doses above 4,000 IU/day long-term require monitoring.

Pair with: vitamin K2 to direct calcium properly (see our Vitamin combinations article). Take with food containing fat for absorption. Magnesium is also a cofactor in vitamin D metabolism.

Test annually: if you supplement, retest 25-OH vitamin D once a year, ideally in late winter when levels are at their lowest, to confirm the dose is working.

What this means for the nōuxx routine

The nōuxx routine contains vitamin D3 at a level appropriate for general support. The dose is selected to complement adequate dietary intake and provide insurance during the lower-sun months without crossing into therapeutic dosing territory.

For women with documented deficiency, autoimmune thyroid conditions, PCOS, or specific clinical indications, higher therapeutic doses may be appropriate under medical guidance. The routine is designed as a baseline; clinical situations may warrant additional targeted supplementation.

Common questions

Should I test my vitamin D?

If you live in Northern Europe, particularly if you spend most of winter indoors, work in an office, have darker skin (which produces vitamin D less efficiently), are postmenopausal, have an autoimmune condition, have PCOS, or have unexplained fatigue or mood symptoms, yes. The test (serum 25-hydroxyvitamin D) is inexpensive and often covered by insurance. Testing in late winter (February to April) gives you the most useful "worst case" reading.

Can I get enough from food?

For most women, no. Dietary sources of vitamin D are limited: fatty fish (salmon, mackerel, sardines), egg yolks, fortified dairy or plant milks, mushrooms exposed to UV light. Hitting 600 to 800 IU from food alone is possible but requires regular consumption of these foods.

Is vitamin D toxicity a real concern?

At reasonable supplementation doses (under 4,000 IU/day), no. Toxicity is rare and typically requires very high doses (>10,000 IU/day) sustained for months. Symptoms include hypercalcemia (kidney stones, calcification of soft tissues). If you are taking more than 4,000 IU/day, periodic testing of both 25-OH vitamin D and serum calcium is reasonable.

What about during pregnancy?

Vitamin D status in pregnancy affects both maternal and fetal outcomes. The German DGE recommends 800 IU/day in pregnancy unless higher doses are clinically indicated. Some clinicians recommend higher doses (1,000 to 2,000 IU/day) particularly in winter pregnancies. Discuss with your maternity care provider.

What about during breastfeeding?

Breast milk is naturally low in vitamin D. Both maternal vitamin D supplementation and direct infant vitamin D supplementation are part of pediatric guidance. The German DGE recommends 800 IU/day for breastfeeding women.

Is winter "vitamin D deficiency" really a problem?

For most adults at Northern European latitudes, 25-OH vitamin D levels drop substantially through winter and rebuild through spring and summer. For some women, summer levels are high enough that the winter dip is still within an adequate range. For many, winter levels drop into the deficient range. Testing reveals which group you are in.

Can vitamin D help fertility?

Vitamin D status affects ovulation quality, AMH levels, and implantation. For women trying to conceive or with documented vitamin D deficiency, supplementation is part of the evidence base. It is not a fertility treatment in itself, but adequate status supports the systems involved in conception.

Does the type of milk (cow vs plant-based) matter for vitamin D?

Fortified plant-based milks (soy, oat, almond) typically contain similar vitamin D levels per serving to fortified cow's milk. Both can contribute. Unfortified plant milks contain little vitamin D. Read the label.

The bottom line

Vitamin D is functionally a steroid hormone with effects across the menstrual cycle, PMS severity, PCOS, thyroid autoimmunity, fertility, and bone health. For European women, particularly in winter, deficiency is common and the supplementation evidence is reasonable for several specific situations: deficiency-treatment, PCOS, autoimmune thyroid conditions, low-sun lifestyles, and pregnancy.

The single highest-leverage step is testing your 25-OH vitamin D level and supplementing toward the 75 to 125 nmol/L range if you are below it. Use D3 form, pair with K2, take with fat-containing food, retest annually. This is one of the better-evidenced supplementation decisions in women's nutrition.

References

[1] Jukic AMZ, et al. Increasing serum 25-hydroxyvitamin D is associated with reduced odds of long menstrual cycles in a cross-sectional study of African American women. Fertility and Sterility 2016;106(1):172-179.e2. doi.org/10.1016/j.fertnstert.2016.03.004

[2] Singh V, et al. Association between serum 25-hydroxy vitamin D level and menstrual cycle length and regularity: A cross-sectional observational study. International Journal of Reproductive Biomedicine 2021;19(11):979-986. doi.org/10.18502/ijrm.v19i11.9913

[3] Chu C, et al. Relationship Between Vitamin D and Hormones Important for Human Fertility in Reproductive-Aged Women. Frontiers in Endocrinology 2021;12:666687. doi.org/10.3389/fendo.2021.666687

[4] Abdul-Razzak K, et al. Severity of premenstrual symptoms among women with musculoskeletal pain: relation to vitamin D, calcium, and psychological symptoms. Journal of Medicine and Life 2024;17(4):397-405. doi.org/10.25122/jml-2023-0050

[5] Balogh Z, et al. Relations of Insulin Resistance, Body Weight, Vitamin D Deficiency, SHBG and Androgen Levels in PCOS Patients. Biomedicines 2025;13(8). doi.org/10.3390/biomedicines13081803

[6] Trummer C, et al. Vitamin D, PCOS and androgens in men: a systematic review. Endocrine Connections 2018;7(3):R95-R113. doi.org/10.1530/EC-18-0009

[7] Sun W, et al. Vitamin D deficiency in Hashimoto’s thyroiditis: mechanisms, immune modulation, and therapeutic implications. Frontiers in Endocrinology 2025;16. doi.org/10.3389/fendo.2025.1576850

[8] Jiang X, et al. Therapeutic effect of vitamin D in Hashimoto's thyroiditis: a prospective, randomized and controlled clinical trial in China. American Journal of Translational Research 2023;15(10):6234-6241. pubmed.ncbi.nlm.nih.gov/37969187

[9] Prevalence and Predictors of Subclinical Micronutrient Deficiency in German Older Adults: Results from the KORA-Age Study. Nutrients. (also in [Do I need supplements article](https://nouxx.com/blogs/science/do-i-need-supplements)). mdpi.com/2072-6643/9/12/1276/htm

[10] Balachandar R, et al. Relative Efficacy of Vitamin D(2) and Vitamin D(3) in Improving Vitamin D Status: Systematic Review and Meta-Analysis. Nutrients 2021;13(10). doi.org/10.3390/nu13103328

[11] Dragomir RE, et al. The Key Role of Vitamin D in Female Reproductive Health: A Narrative Review. Cureus 2024;16(7):e65560. doi.org/10.7759/cureus.65560

[12] Dastorani M, et al. The effects of vitamin D supplementation on metabolic profiles and gene expression of insulin and lipid metabolism in infertile polycystic ovary syndrome candidates for in vitro fertilization. Reproductive Biology and Endocrinology : RB&E 2018;16(1):94. doi.org/10.1186/s12958-018-0413-3

[13] Krysiak R, et al. Sexual Function and Depressive Symptoms in Young Women with Euthyroid Hashimoto's Thyroiditis Receiving Vitamin D, Selenomethionine and Myo-Inositol: A Pilot Study. Nutrients 2023;15(12). doi.org/10.3390/nu15122815

[14] Lebiedziński F, Lisowska KA. Impact of Vitamin D on Immunopathology of Hashimoto's Thyroiditis: From Theory to Practice. Nutrients 2023;15(14). doi.org/10.3390/nu15143174