Vitamin D and Perimenopause: Testing, Dosing, and Why Most Women Are Low

Why Vitamin D Is Critical During Perimenopause

Vitamin D isn't just a bone nutrient. It's actually a hormone, and like estrogen, it declines with age and behaves differently once your hormonal landscape starts shifting. Vitamin D receptors are present in nearly every tissue in the body, including the brain, immune cells, muscle tissue, and the gut. When vitamin D levels drop, you feel it everywhere, not just in your skeleton.

The relationship between estrogen and vitamin D metabolism is close. Estrogen helps activate vitamin D in the kidneys, so as estrogen declines in perimenopause, your body becomes less efficient at converting the vitamin D you make or consume into its active, usable form. This creates a double problem: you need more vitamin D at exactly the time your body is less capable of using what it gets. It's one of the reasons vitamin D insufficiency is so prevalent in perimenopausal women even in sunny climates.

Low vitamin D in perimenopause is associated with worse bone density loss, more severe mood disturbances, increased fatigue, disrupted sleep, higher inflammation markers, and a less functional immune system. Some research has also linked low vitamin D to greater hot flash frequency. None of these are proven cause-and-effect relationships in every case, but the associations are consistent enough that optimizing your vitamin D status is one of the higher-leverage nutritional moves you can make during this transition.

How to Test Your Vitamin D Levels

The test you want is 25-hydroxyvitamin D, also written as 25(OH)D. This is the storage form of vitamin D in your blood and the best measure of your overall vitamin D status. It's a standard blood test that your doctor can order, and it's often covered by insurance as part of a routine checkup, though coverage varies. You can also order it yourself through direct-to-consumer lab services if you want to check without a doctor visit.

The 1,25-dihydroxyvitamin D test is different. It measures the active form and is not useful for assessing overall status. In fact, levels of active vitamin D can be normal or even elevated when storage levels are low, which is counterintuitive and can give a falsely reassuring picture. Make sure the test ordered is specifically 25(OH)D.

Timing your test matters a little. Vitamin D levels tend to be at their seasonal peak in late summer and early fall in the northern hemisphere, and at their lowest in late winter and early spring. Testing in late winter gives you a picture of your true floor. Testing in fall gives you your peak. Many functional medicine practitioners suggest testing twice a year to understand your full seasonal range.

What 'Normal' Means vs. What's Actually Optimal

Conventional laboratory ranges typically flag a vitamin D level below 20 ng/mL (or 50 nmol/L) as deficient. Levels between 20 and 30 ng/mL are usually called insufficient. Most labs consider anything above 30 ng/mL to be sufficient and don't raise any flags. But a growing body of research suggests that optimal functioning, especially for bone, immune, and mood outcomes, happens at levels between 40 and 60 ng/mL.

This gap between "not deficient" and "optimal" is where a lot of perimenopausal women live. A level of 28 ng/mL will come back as a normal result on your lab report, but it may be well below the range where your bones, mood, and immune system work best. This is worth discussing with your doctor, and it helps to come to that conversation knowing that the research on optimal levels generally supports a higher target than the minimum threshold labs use.

Very high levels above 100 ng/mL can cause toxicity, primarily by raising calcium levels too high. This is why supplementing with extremely high doses without testing isn't a good idea. But in clinical practice, vitamin D toxicity is rare and almost always tied to very high supplemental doses over extended periods. The more common concern is the large number of women sitting in the insufficient-to-low-normal range who are told their levels are fine.

D3 vs. D2: Why the Form You Take Matters

Vitamin D comes in two supplemental forms: D3 (cholecalciferol) and D2 (ergocalciferol). D3 is the form your skin naturally produces from sunlight, and it's significantly more potent at raising and maintaining blood levels than D2. Multiple head-to-head comparisons have found that D3 raises 25(OH)D levels roughly 87 percent more effectively than the same dose of D2 over time.

D2 is still used in some prescription formulations (the 50,000 IU weekly prescription that some doctors prescribe is typically D2) and in vegan supplements, since D3 has historically been derived from lanolin in sheep's wool. However, vegan D3 derived from lichen is now widely available and performs comparably to standard D3. If you're plant-based, look specifically for "vegan D3" or "lichen-derived D3" on the label.

When you see vitamin D added to foods like dairy, plant milks, and cereals, it's often D2. This is fine as part of your overall intake but shouldn't be your primary supplemental source if you're trying to correct a deficiency. For supplementing, choose D3 in an oil-based capsule or softgel rather than a dry tablet, because vitamin D is fat-soluble and absorbs substantially better when taken with fat.

The Cofactors You Need: K2 and Magnesium

Vitamin D doesn't work alone. Two cofactors are especially important and often missing from discussions about vitamin D supplementation: vitamin K2 and magnesium.

Vitamin K2 (specifically the MK-7 form) acts as a traffic director for calcium, guiding it into bones and teeth rather than allowing it to deposit in soft tissues like arteries and kidneys. When you take vitamin D, it increases calcium absorption. Without enough K2, that extra calcium can end up in places you don't want it. Most experts who work extensively with vitamin D now recommend taking K2 alongside it, with 90 to 200 micrograms of MK-7 being a common range. K2 is found in fermented foods like natto, gouda, and brie, but most people don't eat enough of these to get meaningful amounts through diet alone.

Magnesium is essential for converting vitamin D into its active form. Without adequate magnesium, you can supplement vitamin D and still not see it working as expected because the enzyme that activates it requires magnesium. This is a practical and common reason why vitamin D supplementation doesn't raise blood levels as expected in some people. If you're taking vitamin D and your levels aren't moving, checking your magnesium intake is a logical next step. These two supplements work so closely together that taking them simultaneously is often the most sensible approach.

Sun Exposure: How Much Is Enough and What Gets in the Way

Your skin produces vitamin D3 when ultraviolet B rays from the sun hit it directly. No sunscreen, no glass, and no cloud cover in between. The amount you produce depends on your latitude, the time of day, the season, your skin tone, your age, and how much skin is exposed. In general, fair-skinned people at a latitude similar to Atlanta or Los Angeles can produce meaningful vitamin D from about 10 to 20 minutes of midday summer sun exposure on face, arms, and legs. Darker skin tones require longer exposure for the same output.

Above roughly 37 degrees north latitude (roughly the line from San Francisco to Richmond, Virginia), UVB rays are too weak to produce meaningful vitamin D from October through March, regardless of how sunny it feels. This is why sun exposure-based vitamin D production is simply not a year-round solution for most Americans and Europeans.

Age also matters. Skin becomes less efficient at synthesizing vitamin D as you get older, and perimenopausal women in their 40s and 50s produce significantly less vitamin D per minute of sun exposure than a 20-year-old would. This progressive decline in synthesis efficiency is another reason supplementation becomes increasingly important as you move through midlife.

How Much to Supplement and When

For a woman with a blood level in the 20 to 30 ng/mL range and a goal of reaching 40 to 60 ng/mL, a daily supplement of 2,000 to 3,000 IU of D3 is a reasonable starting point. Some women need 4,000 IU to maintain optimal levels, especially in winter or if they have darker skin, higher body fat, limited sun exposure, or absorption issues. The widely cited advice to take just 600 to 800 IU is the Recommended Dietary Allowance for preventing deficiency, not for achieving optimal levels, and it's often not enough for perimenopausal women with documented insufficiency.

Retesting after 2 to 3 months of consistent supplementation tells you whether your dose is working. If levels haven't moved much, you may need to increase the dose, take it with a higher-fat meal, or check your magnesium status. Once you reach your target range, retesting annually or twice yearly to stay in range is good practice.

Taking vitamin D with your largest meal of the day, especially one that contains some fat, meaningfully improves absorption. Studies have found absorption increases by 50 percent or more when D3 is taken with a fat-containing meal versus a fasting state. Breakfast with eggs, dinner with salmon, or lunch with avocado are all good timing options.

Vitamin D and Bone Health During the Perimenopause Transition

Bone loss accelerates sharply in the years around the final menstrual period, with some women losing 2 to 3 percent of bone density per year in early perimenopause. Estrogen was protecting your bones, and as it fluctuates and declines, that protection fades. Vitamin D is a critical partner for calcium in maintaining bone density during this window.

The relationship between vitamin D and bone is more nuanced than simply "take D for strong bones." Vitamin D enables calcium absorption in the gut. Without adequate D, you can consume plenty of calcium and still absorb only a small fraction of it. A study from the Women's Health Initiative found that women who were consistently deficient in vitamin D had significantly faster bone density loss at the hip and spine. Conversely, optimizing D levels as part of a comprehensive bone health strategy, alongside calcium, protein, weight-bearing exercise, and in many cases hormone therapy, is associated with better bone density outcomes.

For perimenopausal women who haven't had a bone density scan (DEXA scan), asking for a baseline measurement between 45 and 50 is a reasonable conversation to have with your doctor. Having baseline data makes it possible to track whether your bone health strategies are working. Vitamin D is just one part of that picture, but it's a foundational one.

Medical Disclaimer

The information in this article is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Vitamin D supplementation, especially at higher doses, should ideally be guided by blood testing and a healthcare provider's input. High-dose vitamin D supplementation can affect calcium levels and may interact with certain medications including thiazide diuretics and some heart medications. Always consult your doctor before starting or significantly changing any supplement regimen.