Researchers wanted to know if people with low vitamin D are more likely to have thyroid problems. They compared 53 people with a mild thyroid condition called subclinical hypothyroidism to 53 healthy people and measured their vitamin D levels. Surprisingly, they found that vitamin D levels were similar in both groups, suggesting that low vitamin D might not be directly connected to this type of thyroid problem. However, the study was relatively small and done in one location, so more research is needed to confirm these findings.

The Quick Take

  • What they studied: Whether people with a mild thyroid condition (where the thyroid isn’t working quite right but not severely) have lower vitamin D levels than people with healthy thyroids.
  • Who participated: 106 adults from Iran, average age around 35 years old. Half had subclinical hypothyroidism (a mild thyroid condition), and half were completely healthy.
  • Key finding: People with the mild thyroid condition had slightly lower vitamin D levels (24 ng/mL) compared to healthy people (27 ng/mL), but this difference was too small to be meaningful or statistically significant (P = .10).
  • What it means for you: If you have a mild thyroid condition, low vitamin D might not be the main cause. However, this is just one study in one location, so you shouldn’t ignore vitamin D health. Talk to your doctor about whether vitamin D testing or supplementation makes sense for you personally.

The Research Details

This was a case-control study, which is like comparing two groups of people to see what’s different about them. Researchers recruited 53 people with subclinical hypothyroidism (a condition where the thyroid isn’t producing enough hormone, but not severely) and 53 healthy people without thyroid problems. They measured everyone’s vitamin D levels, thyroid hormone levels, and recorded basic information like age, weight, and height. All participants were patients at an endocrinology clinic in Iran. The researchers then used statistical software to analyze whether the two groups had meaningfully different vitamin D levels.

Case-control studies are useful for investigating whether two health conditions might be related, but they have some limitations. They’re faster and cheaper than some other study types, but they can’t prove that one condition causes another—only that they might be associated.

Understanding whether vitamin D deficiency is connected to thyroid problems is important because both conditions are common, especially in certain parts of the world like Iran. If they were strongly connected, doctors could potentially help thyroid patients by treating their vitamin D deficiency. However, this study suggests the connection might not be as strong as some researchers thought.

This study has moderate reliability. The researchers used proper medical measurements and compared similar-sized groups, which is good. However, the study was relatively small (only 106 people) and conducted in just one hospital in Iran, so the results might not apply to other populations or countries. The lack of a significant difference could mean there truly isn’t a strong connection, or it could mean the study wasn’t large enough to detect one. The fact that vitamin D levels were measured from medical records (rather than specifically collected for this study) is also a limitation.

What the Results Show

The main finding was that vitamin D levels were very similar between the two groups. People with subclinical hypothyroidism had an average vitamin D level of 24 ng/mL, while healthy people had an average of 27 ng/mL. This 3-point difference is quite small and wasn’t statistically significant, meaning it could easily have happened by chance.

Interestingly, when researchers looked at whether vitamin D levels were connected to other factors like age, sex, or body weight, they found no meaningful connections in either group. This suggests that vitamin D deficiency in this population might be caused by other factors, such as sun exposure or diet, rather than thyroid function.

As expected, the researchers did find that people with subclinical hypothyroidism had higher thyroid-stimulating hormone (TSH) levels than healthy people. This is the hallmark of the condition and confirms that the two groups were properly identified.

The study didn’t find any significant differences in how vitamin D related to age, sex, or body mass index in either group. This is somewhat surprising because previous research has sometimes found connections between these factors and vitamin D levels. The lack of correlation in this study might suggest that in this particular population, other factors (like geographic location, sun exposure patterns, or dietary habits) are more important for determining vitamin D levels than these demographic factors.

Some earlier research suggested that vitamin D deficiency might be connected to thyroid problems, which is why researchers wanted to investigate this in Iran, where vitamin D deficiency is very common. However, this study’s findings suggest the connection might be weaker than previously thought, or that the relationship is more complex than a simple cause-and-effect. The results align with some recent research questioning whether vitamin D deficiency is a primary cause of subclinical hypothyroidism, though other studies have found different results.

This study has several important limitations. First, it was relatively small with only 106 participants, which means it might not have been large enough to detect a real but small difference in vitamin D levels. Second, it was conducted in only one hospital in Iran, so the results might not apply to other countries or populations with different sun exposure, dietary patterns, or genetic backgrounds. Third, vitamin D levels were taken from existing medical records rather than being specifically measured for this study, which could introduce errors. Finally, the study design (case-control) can show associations but cannot prove that one condition causes another.

The Bottom Line

Based on this study alone, there isn’t strong evidence that vitamin D deficiency directly causes subclinical hypothyroidism. However, vitamin D is important for overall health, so maintaining adequate vitamin D levels is still recommended. If you have subclinical hypothyroidism, focus on your thyroid treatment as prescribed by your doctor rather than assuming vitamin D supplementation will fix the problem. That said, getting enough vitamin D through sun exposure, diet, or supplements is still a good general health practice. Talk to your doctor about whether vitamin D testing or supplementation is right for you.

This research is most relevant to people with subclinical hypothyroidism who are wondering if vitamin D deficiency is contributing to their condition. It’s also important for doctors treating thyroid patients in areas with high vitamin D deficiency rates. However, the findings shouldn’t concern people with normal thyroid function—this study doesn’t suggest that low vitamin D causes thyroid problems in healthy people. People in sunny climates or with good dietary vitamin D intake may find this less relevant to their situation.

If you’re considering vitamin D supplementation for thyroid health, don’t expect immediate results. Vitamin D works slowly in the body, and it typically takes several weeks to months to see changes in blood levels. If you’re treating subclinical hypothyroidism, focus on your thyroid medication as prescribed, which usually takes 6-8 weeks to show full effects. Any vitamin D supplementation should be viewed as a general health measure, not a thyroid treatment.

Want to Apply This Research?

  • Track your vitamin D levels quarterly (every 3 months) if you have subclinical hypothyroidism. Record the actual blood test values (in ng/mL) along with the date. Also track your TSH levels at the same intervals to monitor your thyroid condition separately. This helps you and your doctor see patterns over time.
  • If your doctor recommends vitamin D supplementation, use the app to set a daily reminder to take your supplement at the same time each day (like with breakfast). Also log your sun exposure time on sunny days, as this is a natural way to get vitamin D. Track any dietary sources of vitamin D like fatty fish, egg yolks, or fortified milk.
  • Create a long-term tracking dashboard that shows your vitamin D levels, TSH levels, and thyroid medication adherence side-by-side. This helps you see whether changes in vitamin D correlate with changes in your thyroid function over 6-12 months. Share this data with your doctor at regular check-ups to inform treatment decisions.

This study suggests that vitamin D deficiency may not be directly connected to subclinical hypothyroidism, but it is not definitive proof. This research should not be used to replace medical advice from your doctor. If you have been diagnosed with subclinical hypothyroidism or vitamin D deficiency, continue following your healthcare provider’s treatment recommendations. Do not start, stop, or change any medications or supplements without consulting your doctor first. This single study has limitations and should be considered alongside other research and your individual health situation. Always discuss any concerns about your thyroid or vitamin D levels with a qualified healthcare professional.