While vitamin D is better known for its ability to regulate bone metabolism, as well as calcium and phosphorus homeostasis, studies have found it useful for non-skeletal purposes as well.
Vitamin D is said to help protect against or treat autoimmune and heart diseases, cancer, metabolic syndrome and all-cause mortality.
More recently, vitamin D insufficiency was shown to have an inverse association with autoimmune thyroid diseases like Graves' disease and Hashimoto's thyroiditis. Impaired vitamin D signalling has also been observed with regards to thyroid cancers.
This led a researcher from the Dankook University College of Medicine to conduct a review of recent data on the potential of vitamin D in thyroid diseases.
A total of 86 studies were reviewed, and it was found that vitamin D "directly leads to a shift from a pro-inflammatory to a more tolerogenic immune status, which includes diverse effects on T cell subtypes".
Simply put, vitamin D's immunomodulatory properties inhibit the adaptive immune system, thereby enhancing immune tolerance against autoimmune diseases like thyroid.
The review stated that vitamin D-deficient mice had been found to develop persistent hyperthyroidism following immunisation with the thyrotropin receptor, which responds to the thyroid-stimulating hormone. This result was not observed in mice with sufficient vitamin D levels.
Another study on mice showed that administrating a mixture of vitamin D and cyclosporine effectively prevented induced experimental autoimmune thyroiditis (EAT).
Yet another study involving EAT, this time in rats, "prevented and ameliorated the pathological changes of the thyroid gland and corrected thyroid autoantibody production and the cytokine disequilibrium".
Certain clinical studies have found insufficient vitamin D status in autoimmune thyroid diseases or Hashimoto's thyroiditis, suggesting a link between vitamin D deficiency and thyroid autoimmunity.
Vitamin D deficiency was more prevalent among thyroid patients than healthy controls, and reported to be related to the presence of anti-thyroid antibodies. This implied that vitamin D was involved in the pathogenesis of autoimmune thyroid diseases.
Furthermore, "the severity of vitamin D deficiency was correlated with the duration of Hashimoto's thyroiditis, thyroid volume, and antibody levels", meaning vitamin D might play a part in developing Hashimoto's thyroiditis, and / or its progression to hypothyroidism.
On the contrary
However, other studies suggest vitamin D insufficiency or deficiency was not a cause but instead, a consequence of thyroid diseases. Patients are more likely to have low vitamin D intake, malabsorption, and minimal sun exposure and outdoor activity as a result of illness.
Additionally, healthy individuals tend to have higher vitamin D levels and a lower risk of chronic illness. The review therefore stated that it is "difficult to separate the effects of these characteristics from those that may be attributed to" vitamin D levels.
Variability in measuring vitamin D levels, seasonal variations in blood sampling of vitamin D, and varying cut-off levels to determine vitamin D insufficiency or deficiency were also mentioned as possible reasons for the inconsistencies in study results.
Other factors included small sample sizes, study population heterogeneity, and differing diagnosis methods for autoimmune thyroid diseases, Graves' disease, Hashimoto’s thyroiditis, and thyroid cancer.
The review concluded: "Ongoing and future long-term, randomised controlled trials are required to determine whether individuals with low 25(OH)D levels are at increased risk of developing autoimmune thyroid diseases and thyroid cancer, and to provide insight into the efficacy and safety of vitamin D as a therapeutic tool for these thyroid diseases."
Source: International Journal of Molecular Sciences
"The Role of Vitamin D in Thyroid Diseases"
Author: Dohee Kim