How to track your vitamin D over time

Vitamin D is one of the most ordered tests, even though current guidelines say most healthy adults do not need it routinely. It turns up on routine panels, wellness checkups, and direct-to-consumer kits, and it often leads to a supplement and a follow-up test a few months later. That makes it a test many people have several results for, spread over years, labs, and seasons. Read one at a time, those results are hard to make sense of. Read together, with the season and the supplement dose next to each one, they show whether your level actually changed and whether the supplement did what it was meant to.

What the test measures

Your body makes vitamin D in the skin from sunlight and absorbs it from food and supplements. The liver converts it to 25-hydroxyvitamin D, or 25(OH)D, which is the main form circulating in your blood. The kidneys then convert a small amount of that into 1,25-dihydroxyvitamin D, the active hormone.

The test that reflects your vitamin D status is 25(OH)D. It has a half-life of about two to three weeks, so it reflects your sun exposure and intake over the past several weeks, according to a review by Michael Holick.

The other test, 1,25-dihydroxyvitamin D, is a different measurement for a different purpose. Its half-life is only a few hours, and the body holds it steady. When you are low on vitamin D, the parathyroid glands push the kidneys to make more of it, so it can read normal or even high in someone who is deficient. The same review says it “provides no information about vitamin D status.” It is ordered for specific problems, such as some kidney and calcium disorders.

The names are close enough to mix up, and some reports shorten both to “Vitamin D”. Before you put a result in your history, check which one it is. A 1,25 result on a 25(OH)D chart is not a data point. It is a different test.

The numbers on the report

ng/mL and nmol/L

In the United States, 25(OH)D is usually reported in ng/mL. Canada, the UK, Europe, and Australia usually use nmol/L. The numbers are far apart, so a mixed history will look like it jumped by a factor of two and a half at the switch. The NIH Office of Dietary Supplements gives the conversion as:

1 ng/mL = 2.5 nmol/L, and 1 nmol/L = 0.4 ng/mL

The exact factor for total 25(OH)D is 2.496, which makes no practical difference.

The cutoffs disagree

Vitamin D is unusual: there is no agreed answer to what a good level is. Two long-standing positions sit about 10 ng/mL apart, and the more recent guideline declines to name a number at all.

In 2011, the Institute of Medicine (now the National Academies) set intake recommendations that correspond to a 25(OH)D of at least 20 ng/mL (50 nmol/L), which it judged to meet the needs of at least 97.5 percent of people. The NIH Office of Dietary Supplements summarizes that framework as:

The same year, the Endocrine Society published a guideline that defined deficiency as below 20 ng/mL, insufficiency as 21 to 29 ng/mL, and recommended aiming above 30 ng/mL. Many labs adopted that 30 as the bottom of their printed range, which is why a result of 24 ng/mL can be flagged low on one report and called adequate by the NIH framework.

In 2024 the Endocrine Society replaced that guideline. The new guideline on vitamin D for the prevention of disease says the Society “no longer endorses the target 25(OH)D level of 30 ng/mL,” and no longer endorses any specific levels to define sufficiency, insufficiency, or deficiency. It found no clear evidence for an optimal level for preventing disease. The 2024 guideline applies to generally healthy people. It does not cover people with conditions that change how the body handles vitamin D, such as malabsorption or chronic kidney disease, or people who already have a reason for testing or treatment.

So when your report prints a range, it reflects the lab’s choice of framework, not a settled fact. If you and your clinician have settled on a target, chart against that.

How often it is tested

For most healthy adults, the current answer is not routinely. The 2024 Endocrine Society guideline suggests against routine 25(OH)D testing in healthy adults of every age group it considered, which is a change from its 2011 advice. The U.S. Preventive Services Task Force found the evidence insufficient to recommend for or against screening in adults without symptoms.

Testing is still common, and still reasonable, in other situations: a condition that affects absorption, such as bariatric surgery or celiac disease, kidney or liver disease, bone problems, medicines that affect vitamin D, or a previous low result being treated. In those cases your clinician sets the schedule.

That leaves many people with an uneven history: a few results years apart, then a cluster after a low result and a new supplement. Each one is worth keeping. A result from a job physical or an old portal may be your only reading from before you started supplementing. How to get all of your medical records covers requesting the older ones.

What counts as a real change

Vitamin D varies from draw to draw even in the same season. A European multicenter study that sampled 91 healthy people every week for ten weeks put the within-person variation at about 6 percent, after removing outliers, and noted that levels were still rising through the spring, so participants were never truly steady. Other studies in the EFLM database, cited in the same paper, report within-person variation from about 7 to 21 percent. Add the test’s own imprecision on top, and two results from the same lab can differ by a fifth or so with nothing real behind it. On a level of 30 ng/mL, anything from about 25 to 35 can be ordinary noise.

A few questions help sort it out:

• Is the change bigger than your usual spread? If your last four results ran from 28 to 34, a 31 is not news. A 19 is.
• Were both draws in the same season? See the next section. A winter result next to a summer result is not a fair comparison.
• Did the lab or the method change? Assays differ more for vitamin D than for most tests.
• Did your supplement dose change, and how long ago? A new dose takes months to show its full effect.

Things that move your vitamin D

Season

This is the biggest one for most people. Sunlight drives most of the vitamin D your skin makes, and the level in your blood lags behind it. An analysis of 3.4 million U.S. samples found that 25(OH)D peaks in late summer, around August, and bottoms out in late winter, around February, about two months behind the sun. The size of the swing depends on where you live, how much time you spend outside, your skin, and whether you take a supplement.

For tracking, this means a February result and an August result can differ by a lot without anything about you having changed. When you compare two results, compare the same season if you can. A chart with several years on it will often show a wave. That wave is the pattern, not a problem.

Supplements and how long they take

A supplement raises 25(OH)D gradually. In a classic dosing study, each additional microgram (40 IU) of vitamin D3 a day raised the steady-state level by about 0.7 nmol/L, which is roughly 0.3 ng/mL per 40 IU, or about 0.7 ng/mL per 100 IU. People respond differently, so treat that as a rough average, not a prediction.

The bigger point for tracking is time. Retesting about three months after starting is common practice, but a 2025 study of older adults taking 700 IU a day found that normal-weight participants had reached a stable level by six months, while overweight and obese participants were still rising between six and twelve months. A result at three months may show only part of the effect. Did the medication actually do anything? covers reading a before-and-after fairly, and the same logic applies to a supplement.

Remember to count every source. A multivitamin, a calcium-plus-D tablet, and fortified foods all add up, and a dose change in any of them can move the next result.

The lab and the assay

Vitamin D is hard to measure well, and labs have not always agreed. In one comparison described in the Holick review, the same people’s samples averaged anywhere from about 17 to 36 ng/mL depending on the lab, enough to move someone from “deficient” to “fine”. Most labs now use automated immunoassays. Some use liquid chromatography with tandem mass spectrometry (LC-MS/MS), often considered the more specific method.

Things have improved. The CDC runs a Vitamin D Standardization-Certification Program that checks tests against a reference method and certifies those that meet its accuracy and precision criteria. Still, a switch of labs or methods can produce a step in your history that has nothing to do with your blood. Note the lab with every result.

D2, D3, and the total

Supplements come in two forms: D3 (cholecalciferol) and D2 (ergocalciferol). Some prescription vitamin D is D2. LC-MS/MS methods often report 25(OH)D2 and 25(OH)D3 separately, plus a total. The Holick review says that physicians “only need to be aware of the total 25(OH)D level.” Some immunoassays pick up D2 less completely than D3, so if you take D2, the lab’s method matters more than usual.

For tracking, chart the total. If a report lists D3 alone, it leaves out any D2, and it should not share a line with totals from other reports.

Other things

The NIH fact sheet lists groups more likely to run low, including people with darker skin, people with obesity, people with limited sun exposure, and people with conditions or surgery that reduce fat absorption. None of these make a result wrong. They are context worth noting, because a change in one of them, such as weight loss after surgery, can move your level on its own.

What to record with each result

For each vitamin D result, keep:

The test goes by many names: Vitamin D, Vitamin D 25-Hydroxy, 25-Hydroxyvitamin D, 25(OH)D, 25-OH Vitamin D, Vitamin D Total, and Calcidiol. Pick one name and use it everywhere, or your history will split into short, separate lines. Keep 1,25-dihydroxyvitamin D (also written 1,25(OH)2D or calcitriol) under its own name.

Charting it

A simple line chart is enough. A few choices make it honest:

• Space points by date, and look at the months. A chart that shows the seasons will often explain a drop you were worried about.
• Draw your target, not just the lab range. If your clinician is happy with 20 ng/mL and the lab prints 30 to 100, a lab-range chart will flag results that are fine for you.
• Mark supplement starts, stops, and dose changes, and give each one months, not weeks, before judging it.
• Mark lab switches. A step that lines up with a new lab is more likely the lab.
• Keep the 1,25 test off the chart. It answers a different question.

How to track your lab results over time has the general method for any test. If you are also tracking iron, the ferritin post covers another test that often comes with a supplement and a retest.

Tracking vitamin D in HealthViewer

HealthViewer is a lab results tracker that runs on your own computer. It imports vitamin D results from lab report PDFs, MyChart downloads, and direct connections to your health system, and files Vitamin D, 25-Hydroxyvitamin D, 25(OH)D, 25(OH) Vitamin D, and Vitamin D 25-OH Total under one test. Results in nmol/L are converted so they share one chart with results in ng/mL.

Some names are kept separate on purpose. 25-OH D3 alone is not filed with the D2 plus D3 total, and 1,25-dihydroxyvitamin D is treated as the different test it is. If your lab uses another spelling, such as “Vitamin D, 25-Hydroxy”, you can combine it with the others using Merge… in the Test Catalog.

In Trends, the Vitamins tag and the Vitamin Panel filter pull vitamin D together with B12 and folate. Add your supplement in Medications, and medication markers show on the chart where it started or stopped. A journal note on the date of a lab switch, a dose change, or a winter spent indoors can be shown on the chart too. Set your own target as a custom range in the Test Catalog, and the chart shades your target instead of the lab’s range.

Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.

This post is general information, not medical advice. Talk to your clinician about whether you need vitamin D testing, what level to aim for, and before starting or changing a supplement.