Perhaps the most commonly known function of vitamin D is its role in keeping your bones strong by helping your body absorb calcium. However, vitamin D has other roles, such as regulating the immune system, and it may even play a role in keeping your thyroid functioning properly. Your thyroid is a small gland that you're likely not going to hear much about until it malfunctions. It produces hormones that have a wide variety of physiological roles, including regulating your metabolism. Evidence suggests that low vitamin D levels may have a negative effect on thyroid function.

Where You Get It

Your skin produces vitamin D in response to sunlight exposure, and you get it from foods in your diet, such as:

• salmon
  
• tuna
  
• egg yolks
  
• cheese
  
• foods fortified with vitamin D

The recommended daily intake is 600 international units per day
* according to the Office of Dietary Supplements 1. Science is still unraveling the effects of vitamin D deficiency. Bone problems, such as rickets, are widely acknowledged, but research indicates that vitamin D deficiency may cause other problems.

Hypothyroid Patient Study

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Researchers conducted a study to examine the prevalence of vitamin D deficiency in patients with hypothyroidism 2. They found that vitamin D levels were significantly lower in those with hypothyroidism, compared with healthy adults. They also found that the degree of vitamin D deficiency was linked to the severity of hypothyroidism -- the more severe the hypothyroidism, the lower the vitamin D levels. The study was published in the November 2013 issue of the "International Journal of Health Sciences. 2"

Link to Hypothyroidism

Autoimmune thyroiditis -- an immune disorder -- is the most common cause of hypothyroidism, which occurs when your immune system mistakes your thyroid for foreign tissue and produces antibodies against it. Researchers found that patients with autoimmune thyroiditis had significantly low vitamin D and that it correlated with thyroid antibodies 3. They concluded that vitamin D deficiency may be involved in hypothyroidism and that the results warranted a recommendation for vitamin D supplementation. The study was published in the May 2011 issue of the journal "Cellular and Modular Immunology."

Health and Supplementation

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There appears to be a link between low vitamin D and hypothyroidism, but more study is needed to draw a solid conclusion. If you have hypothyroidism, ask your doctor to perform a vitamin D test to check your level and discuss whether he recommends supplementation. Avoid supplementing vitamin D without your doctor's guidance.