Her vitamin D level came back at 9.71. The reference range on the lab report said normal was above 20.
So she was technically deficient. But here’s the thing: a level above 20 is what conventional medicine calls acceptable. A level of 9.71 was so far below even that low bar that no one could argue she was fine.
She started coming in once a week for an intramuscular injection. Every time she came in, she left feeling noticeably better. More energy. Mood that lifted. A sense that she was no longer dragging through her days.
After two and a half months, her level had risen from 9.71 to 23. Still not optimal. Still far below where it should be. But already making a visible difference in how she functioned every day.
What “Normal” Actually Means
There is a significant gap between what conventional medicine calls a normal vitamin D level and what functional medicine targets
A conventional provider looks at a vitamin D level of 22 or 23 nanograms per milliliter and calls it normal. No intervention needed. Lab value within range.
A functional medicine provider looks at that same number and sees a deficiency. Not a technical deficiency by the reference range. A functional one. A level at which vitamin D cannot do its job in the body.
The functional medicine target for serum vitamin D is 60 to 80 nanograms per milliliter. Some practitioners target even higher. Toxicity does not typically occur until levels approach 150 nanograms per milliliter, which is extremely difficult to reach through supplementation under clinical guidance
The gap between 22 and 60 is the difference between “your labs look fine” and actually having enough vitamin D to support the systems that depend on it.
What Vitamin D Actually Does
Vitamin D is not just a bone health supplement. It affects thyroid function. It has a direct role in neurotransmitter metabolism, which is how your brain produces and responds to serotonin and dopamine. It regulates inflammation. It supports immune function.
Seasonal affective disorder, the depression and low mood that many people experience through fall and winter, is partially connected to declining vitamin D levels as sunlight hours shorten. Optimizing vitamin D going into the dark months reduces the severity for many patients.
Low vitamin D is connected to fatigue, brain fog, and a low-grade malaise that can be hard to distinguish from depression. It affects bone mineral density. It impairs immune response. It changes how the thyroid functions.
None of these effects are dramatic or sudden. They accumulate quietly over time. A person running a vitamin D of 22 doesn’t collapse. They just feel worse than they could, in ways they’ve started to accept as normal.
Why So Many People Are Deficient
About 90% of new patients at functional medicine practices present with at least one significant vitamin or mineral deficiency. Vitamin D is consistently among the most common.
Geography matters. People living in the northern United States and Canada are not getting enough sun exposure for a significant portion of the year to support adequate vitamin D synthesis through the skin. Maine in January is not the same as Florida in July.
Diet is a factor too. The food sources of vitamin D are limited: fatty fish like salmon, mackerel, and sardines. Fortified dairy products contain some, but fortified milk is not an efficient way to maintain clinical vitamin D levels. Most people are not eating fatty fish twice a day.
Gut health affects absorption. Even if a person is taking vitamin D supplements, if their gut is inflamed or their microbiome is out of balance, absorption may be compromised. The supplement goes in, but not all of it gets used.
Stress and chronic illness burn through vitamins faster. A nervous system running in overdrive consumes nutrients at an accelerated rate. Someone under sustained physical or psychological stress is depleting what they have faster than they’re replenishing it.
The MTHFR Factor
There is a genetic variant called MTHFR that affects how certain vitamins are processed in the body, particularly B vitamins. People with this variant cannot properly convert standard B vitamins into usable form. They can take a high-quality B12 supplement and still be functionally B12-deficient because of how their genetics handle the conversion step.
Taking methylated forms of B vitamins, already in the usable form rather than requiring conversion, bypasses this issue.
MTHFR also affects estrogen metabolism, not just energy and mood. This is relevant for women managing hormonal health or those dealing with conditions that have a hormonal component.
Most people don’t know they have this variant. Standard blood panels don’t test for it. But it is testable, and it changes what supplementation looks like for people who have it.
Minerals: The Deficiency Nobody Talks About
Americans are more deficient in minerals than vitamins. The reason is simple: minerals come primarily from plants, and most Americans don’t eat enough of them.
Magnesium is the most underappreciated example. It fuels the nervous system and the adrenal system. It supports muscle function. It’s involved in hundreds of enzymatic reactions in the body. Chronic stress burns through it rapidly.
Someone who is under sustained stress, who sleeps poorly, who has muscle tension and anxiety and fatigue, is very likely magnesium-deficient. They may have been for years.
Selenium and chromium are less familiar but equally important. Calcium is well-known but often taken in forms that the body doesn’t use effectively. The dairy-for-calcium idea has been so thoroughly marketed that most people don’t know calcium also comes from leafy greens, almonds, and legumes.
Why Testing Changes Everything
A multivitamin is not a substitute for testing. It may help someone who has a generally poor diet. It doesn’t tell you what specifically is low, how low it is, or why.
Testing gives you actual data. It tells you whether you’re deficient, suboptimal, or fine. It tells you whether the deficiency is primarily dietary or whether there’s a gut absorption issue or genetic methylation problem driving it. That information changes what treatment looks like.
Someone who takes 2,000 IU of vitamin D daily might be maintaining a level of 28. That’s “normal” by conventional standards and genuinely inadequate for what vitamin D is supposed to do. Without testing, they’d never know.
Someone who is iron deficient because of heavy menstrual periods needs iron replacement. Someone who is iron deficient because of an undiagnosed gastrointestinal bleed needs something completely different first. The deficiency looks the same on the surface. The causes require different responses.
Getting specific with testing allows treatment to be specific. And specific treatment, targeted to actual deficiencies and their causes, works faster and more reliably than guessing with general supplementation.
The Connection to Sex Hormones
Cholesterol is required to synthesize sex hormones. Testosterone, estrogen, progesterone. Patients who are on very high-dose statin medications, reducing cholesterol significantly, may be experiencing downstream effects on sex hormone production. Low libido, fatigue, poor stress tolerance, mood instability, all can have this as a contributing cause.
Vitamin D supports hormone production pathways. B12 and magnesium support the nervous system that regulates them. Iron supports the energy production needed to feel like a functional human being.
These nutrients are not isolated. They interact with each other and with hormonal systems in ways that don’t show up when you look at any single lab value in isolation. Seeing the whole picture requires looking at the whole picture.
What to Do With This
If your doctor has told you your labs look normal but you’re still fatigued, still foggy, still not feeling right, consider whether “normal” means adequate for optimal function.
A vitamin D of 26 is technically within conventional reference ranges. It is also likely to be contributing to impaired neurotransmitter metabolism, elevated inflammation, and reduced immune function.
The path forward is testing that includes micronutrient levels, looking at actual functional targets rather than just reference ranges, and addressing both the deficiency and why it exists.
That might mean dietary changes to improve absorption. It might mean treating an underlying gut issue. It might mean weekly injections for six weeks followed by maintenance oral supplementation. It depends on what the testing actually shows.
What it almost certainly doesn’t mean is that everything is fine just because the number falls within a reference range.
About the Author: This article was written by the clinical education team at Med Matrix, a functional medicine clinic in South Portland, Maine. Med Matrix serves over 3,000 patients with a provider team that specializes in root-cause testing, hormone optimization, and personalized treatment plans.
