Researchers gave eight healthy adult male dogs vitamin D supplements for six weeks to see if it affected their thyroid gland. The thyroid is a small gland that controls many body functions like energy and metabolism. The study found that vitamin D supplements increased certain thyroid hormones in the dogs’ blood and decreased others. While these changes were measurable and statistically significant, the dogs remained healthy throughout the study. This research suggests vitamin D might influence how the thyroid works, which could be important for understanding how to help dogs with thyroid problems in the future.

The Quick Take

  • What they studied: Whether giving dogs vitamin D supplements would change how their thyroid gland works and the hormones it produces
  • Who participated: Eight healthy adult male dogs received daily vitamin D supplements for six weeks. Blood samples were taken at the start and then at weeks 2, 4, and 6 to measure thyroid hormones
  • Key finding: Vitamin D supplements increased one main thyroid hormone (T4) in the dogs’ blood and decreased another hormone (TSH) that tells the thyroid what to do. These changes happened gradually over the six weeks
  • What it means for you: If you have a dog, this suggests vitamin D might affect thyroid function. However, this was a very small study in healthy dogs, so talk to your veterinarian before giving your dog vitamin D supplements, especially if your dog has thyroid problems

The Research Details

This was a straightforward experiment where researchers gave eight healthy adult male dogs vitamin D3 supplements at a dose of 50 IU per kilogram of body weight each day for six weeks. The researchers took blood samples from the dogs on day 0 (before starting), day 14, day 28, and day 42 to measure different thyroid hormones and see how they changed over time.

The thyroid hormones they measured included T4 (the main hormone the thyroid makes), free T4 (the active form that the body can use), T3 (another thyroid hormone), free T3 (the active form), and TSH (a hormone that tells the thyroid to work). By measuring these at different time points, the researchers could track exactly how vitamin D affected the thyroid system.

The researchers used statistical analysis to determine whether the changes they saw were real and meaningful, not just random variation. They looked at whether changes were significant at each time point compared to the starting point.

This research approach is important because it allows researchers to see how a supplement affects the body over time in a controlled way. By taking multiple blood samples throughout the study, they could see when changes started happening and how they progressed. This helps distinguish between temporary changes and lasting effects. Understanding how vitamin D affects the thyroid in healthy dogs is a necessary first step before studying dogs with thyroid problems.

This study has some important limitations to consider: it included only eight dogs, all of which were healthy adult males, so results may not apply to female dogs, puppies, or older dogs. The study was relatively short (six weeks), so we don’t know what happens with longer-term supplementation. The small sample size means the findings are preliminary and need to be confirmed in larger studies. However, the researchers used proper statistical methods and measured multiple thyroid markers, which strengthens the reliability of what they found.

What the Results Show

After six weeks of vitamin D supplementation, the dogs showed clear changes in their thyroid hormones. The main thyroid hormone T4 increased significantly, with the highest levels appearing at weeks 4 and 6 compared to the starting point. Free T4 (the active form the body uses) also increased, with the biggest difference by week 6.

At the same time, TSH levels decreased throughout the study. TSH is like a messenger that tells the thyroid to produce more hormones, so lower TSH suggests the thyroid was working differently. This pattern—higher T4 with lower TSH—suggests the thyroid gland itself was responding to the vitamin D, not just the pituitary gland that controls it.

Interestingly, T3 and free T3 (another thyroid hormone) did not change significantly, suggesting vitamin D specifically affects T4 production rather than T3. The ratio of free T3 to free T4 decreased over time, meaning the balance between these two hormones shifted.

The ratio of T3 to T4 did not change significantly, but the ratio of free T3 to free T4 did decrease by week 6. This suggests that while total T3 stayed the same, the proportion of active T3 relative to active T4 decreased. This shift in hormone balance could have biological importance, though the study didn’t measure whether the dogs experienced any physical changes from these hormonal shifts.

Previous research has shown that vitamin D affects many body systems beyond just bone health, including immune function and hormone regulation. This study adds to that knowledge by showing vitamin D can influence thyroid hormones specifically. The finding that vitamin D increases T4 while decreasing TSH is consistent with how other factors that stimulate the thyroid work. However, most previous research on vitamin D and thyroid has focused on humans, so this dog study provides new information about how this relationship works in animals.

The study had several important limitations: Only eight dogs were studied, all healthy adult males, so results may not apply to female dogs, older dogs, or sick dogs. The study lasted only six weeks, so we don’t know if these changes continue, reverse, or get stronger with longer supplementation. The study didn’t measure whether these thyroid changes caused any actual physical effects on the dogs’ health, energy, or metabolism. The dose used (50 IU/kg per day) may not be typical for pet dogs, so results may not apply to standard supplementation practices. Finally, there was no control group of dogs that didn’t receive vitamin D, so we can’t be completely certain the changes were caused by the vitamin D rather than other factors.

The Bottom Line

Based on this preliminary research, vitamin D supplementation appears to influence thyroid hormone levels in healthy dogs. However, because this is a very small study, the recommendation is cautious: Do not start giving your dog vitamin D supplements without consulting your veterinarian first. If your dog has thyroid problems, discuss with your vet whether vitamin D supplementation might be helpful, as this research suggests it could potentially affect thyroid function. For healthy dogs, routine vitamin D supplementation beyond what’s in regular dog food is not recommended based on this single study.

Veterinarians and dog owners with dogs that have thyroid problems (especially hypothyroidism, where the thyroid doesn’t work well enough) should pay attention to this research. Dog owners considering vitamin D supplements for their pets should definitely consult their veterinarian first. This research is less relevant for people with human health questions, as dogs’ bodies work differently than human bodies. Researchers studying thyroid function and vitamin D should also find this interesting as a foundation for future studies.

In this study, changes in thyroid hormones began appearing within two weeks and continued to increase through week six. If similar effects occur in other dogs, you might expect to see measurable changes in thyroid hormones within 2-4 weeks of starting supplementation. However, whether these hormonal changes translate to noticeable physical changes in your dog (like energy level or weight) would take longer to observe and would vary by individual dog.

Want to Apply This Research?

  • If your veterinarian recommends vitamin D supplementation for your dog, track the daily dose given (in IU or mg) and note the date and time. Also record any observable changes in your dog’s energy level, appetite, weight, or coat condition on a weekly basis
  • Work with your veterinarian to establish a consistent daily vitamin D supplementation routine for your dog if recommended. Set a daily reminder to give the supplement at the same time each day, and use the app to log when you’ve given it. This helps ensure consistent dosing and makes it easy to report to your vet
  • Schedule thyroid blood tests with your veterinarian at regular intervals (such as every 4-6 weeks) if your dog is on vitamin D supplementation. Use the app to track test dates and results, noting any changes in T4, free T4, and TSH levels over time. Also monitor and log observable health changes like energy, appetite, and weight to correlate with hormonal changes

This research is preliminary and was conducted in only eight healthy dogs. It should not be used as a basis for treating your dog without veterinary guidance. Vitamin D supplementation can be harmful at high doses and may interact with other medications or health conditions. Always consult with your veterinarian before starting any supplement for your dog, especially if your dog has thyroid problems, kidney disease, or is taking other medications. This article is for educational purposes only and does not replace professional veterinary medical advice, diagnosis, or treatment.