Researchers combined results from 23 different studies to see if vitamin D levels affect thyroid cancer risk. They found that people with thyroid cancer tend to have lower vitamin D levels than people without thyroid cancer. People with thyroid cancer were also more likely to have vitamin D deficiency. While this connection is interesting, the researchers say we need more carefully designed studies to understand exactly how vitamin D and thyroid cancer are related and whether low vitamin D actually causes thyroid cancer or if something else is going on.

The Quick Take

  • What they studied: Whether people with low vitamin D levels are more likely to develop thyroid cancer compared to people with normal vitamin D levels
  • Who participated: The analysis looked at 23 previous studies involving thousands of people—some with thyroid cancer and some without it—to compare their vitamin D levels
  • Key finding: People with thyroid cancer had noticeably lower vitamin D levels than people without thyroid cancer. People with thyroid cancer were also 33% more likely to have vitamin D deficiency
  • What it means for you: This suggests that maintaining healthy vitamin D levels might be important for thyroid health, but it doesn’t prove that low vitamin D causes thyroid cancer. More research is needed before doctors can make specific recommendations based on this finding

The Research Details

This is a meta-analysis, which means researchers looked at 23 previous studies that had already been done on vitamin D and thyroid cancer. Instead of doing one big new study, they combined the results from all these smaller studies to see if a pattern emerged. The researchers searched three major medical databases (Embase, Web of Science, and PubMed) to find all relevant studies. They only included studies that compared vitamin D levels in people with thyroid cancer to people without thyroid cancer.

The researchers used a special statistical method to combine the results from different studies, even though the studies measured things slightly differently. They looked at how much lower vitamin D levels were in thyroid cancer patients and how much more common vitamin D deficiency was in this group. They also looked at whether the season when vitamin D was measured, the type of study, and other factors affected the results.

A meta-analysis is powerful because it combines evidence from many studies, which gives us a clearer picture than any single study could. By looking at 23 studies together, the researchers could see if the connection between vitamin D and thyroid cancer was consistent across different groups of people and different research methods. This helps us understand whether the finding is reliable or just a coincidence in one or two studies.

This meta-analysis is a solid piece of research because it included many studies and used proper statistical methods. However, the researchers noted that the studies they combined weren’t all done the same way—some measured vitamin D at different times of year, some used different testing methods, and some had different types of control groups. This variation made it harder to draw firm conclusions. The researchers were honest about these limitations and called for future studies to use more standardized methods.

What the Results Show

The main finding was that people with thyroid cancer had significantly lower vitamin D levels compared to people without thyroid cancer. On average, the difference was noticeable enough to be statistically significant, meaning it’s unlikely to be due to chance alone.

Second, vitamin D deficiency (having very low vitamin D) was more common in people with thyroid cancer. People with thyroid cancer were about 33% more likely to have vitamin D deficiency compared to people without thyroid cancer. This means if you looked at 100 people without thyroid cancer, you might find 10 with deficiency, but in 100 people with thyroid cancer, you’d find about 13 with deficiency.

When the researchers looked at different groups of studies separately, they found these patterns held up in most cases. However, when they looked only at studies that measured vitamin D after thyroid cancer surgery, the pattern wasn’t as clear. This suggests that the timing of when vitamin D is measured matters.

The researchers discovered that the season when vitamin D was measured was very important. Vitamin D levels naturally go up in summer and down in winter because of sun exposure. This seasonal variation was a major reason why different studies got somewhat different results. The type of control group used (whether they came from hospitals or the general population) also affected the results, as did the specific method used to test vitamin D levels.

This meta-analysis brings together conflicting evidence from previous studies. Some earlier studies suggested a link between low vitamin D and thyroid cancer, while others found no connection. By combining 23 studies, this analysis shows that overall, there does appear to be a connection—people with thyroid cancer tend to have lower vitamin D. However, the researchers note that this doesn’t necessarily mean low vitamin D causes thyroid cancer; it could be that thyroid cancer causes vitamin D to drop, or both could be related to a third factor.

The biggest limitation is that the studies included weren’t all done the same way. Some measured vitamin D in summer, others in winter, which naturally changes results. Some studies measured vitamin D before cancer diagnosis, while others measured it after, which could give different answers. The studies also used different methods to test vitamin D and different types of comparison groups. Additionally, most studies were observational, meaning they just observed the connection between vitamin D and thyroid cancer without proving one caused the other. The researchers couldn’t determine from this analysis whether low vitamin D actually causes thyroid cancer or if the relationship works differently.

The Bottom Line

Based on this research, maintaining adequate vitamin D levels is generally good for overall health and may be important for thyroid health, though we can’t say for certain it prevents thyroid cancer. Current recommendations suggest getting 600-800 IU of vitamin D daily for most adults, with higher amounts for older adults. You can get vitamin D from sunlight exposure, foods like fatty fish and fortified milk, or supplements. However, don’t take high-dose vitamin D supplements specifically to prevent thyroid cancer without talking to your doctor first, as we don’t have strong enough evidence yet to recommend that. Confidence level: Moderate—the connection exists, but we need more research to understand it fully.

Everyone should care about maintaining healthy vitamin D levels for general health reasons. This research is particularly relevant for people with a family history of thyroid cancer, people living in areas with limited sunlight, or those who spend most of their time indoors. If you’ve been diagnosed with thyroid cancer, talk to your doctor about whether checking your vitamin D level makes sense. People who are pregnant, very young, or have certain medical conditions should talk to their doctor before making changes to vitamin D intake.

If you start improving your vitamin D levels through diet, supplements, or sun exposure, it typically takes several weeks to months to see vitamin D levels rise in blood tests. However, we don’t know how long it would take to see any potential benefit for thyroid cancer risk, or if improving vitamin D levels would actually reduce that risk. This is why more research is needed.

Want to Apply This Research?

  • Track your vitamin D intake sources weekly: record sun exposure time (15-30 minutes daily is typical), servings of vitamin D-rich foods (fatty fish, fortified milk, egg yolks), and any supplements taken. Note the season and your general location to account for seasonal variation.
  • Set a daily reminder to either spend 15-30 minutes in midday sunlight (without excessive burning), eat one vitamin D-rich food, or take a vitamin D supplement if recommended by your doctor. Log this in your app to build consistency and track patterns over time.
  • Every 3 months, note any changes in energy levels, mood, or bone health. If your doctor recommends it, get vitamin D blood levels tested seasonally (spring and fall) to track trends. Use the app to record test results and correlate them with your daily vitamin D habits to see what works best for you.

This research suggests an association between low vitamin D and thyroid cancer but does not prove that low vitamin D causes thyroid cancer. This information is for educational purposes only and should not replace professional medical advice. If you have concerns about thyroid cancer risk, vitamin D levels, or are considering changes to your vitamin D intake, please consult with your healthcare provider or a registered dietitian. Do not start high-dose vitamin D supplementation without medical supervision. If you have been diagnosed with thyroid cancer, work with your medical team regarding all nutritional decisions.