There's increasing evidence to suggest that low levels of vitamin D may play a role in thyroid disorders. For instance, research has suggested a possible link between vitamin D deficiency and autoimmune thyroid disorders, namely Hashimoto's thyroiditis, the most common cause of hypothyroidism (underactive thyroid), and Grave's disease, characterized by an overactive thyroid.﻿﻿

In one study, 72 percent of subjects with autoimmune thyroid disease were deficient in vitamin D, while just under 31 percent of healthy subjects had low levels of D. Similarly, a study of people in Greece with Hashimoto's thyroiditis revealed that more than 85 percent of them had low levels of vitamin D, as well as high levels of anti-thyroid antibodies.﻿﻿

Vitamin D supplementation also has shown promise as a way to help treat thyroid disease. In the Greek study, for example, the Hashimoto thyroiditis patients who were deficient in vitamin D took 1,200 to 4,000 international units (IU) of vitamin D every day for four months, after which time they had significantly lower levels of anti-thyroid antibodies. (The Recommended Daily Allowance, or RDA, for vitamin D is 600 IU; see below for more specifics.)

In yet another study, people with hypothyroidism who took extra vitamin D supplements for 12 weeks had improvements in blood levels of thyroid stimulating hormone (although the extra D did not affect levels of the actual thyroid hormones triiodothyronine, T3, and thyroxine, T4).﻿﻿

Much more research needs to be done before specific guidelines for using vitamin D to prevent or treat thyroid disease are established by the medical community. However, given the growing understanding of how important vitamin D is to overall health, it's worth making sure you're getting enough of it. You might even want to talk to your doctor about testing you for a deficiency, especially if you have a thyroid condition or are at risk for one.

Defining Vitamin D Deficiency

According to some reports, 40% of adults have insufficient levels of vitamin D, although estimates vary. Certain risk factors are associated with lower levels, including race (African American and Hispanic populations have higher rates of vitamin D deficiency), obesity, lack of college education, and lack of daily milk consumption.﻿﻿

Vitamin D levels are measured with a simple blood test called the 25-hydroxyvitamin D test. Results of this test are measured in nanograms per milliliter (ng/mL), which the National Institutes of Health categorizes as follows:

Sources of Vitamin D

The main source of vitamin D is sun exposure—when skin soaks up ultraviolet rays it triggers the production of D. Concerns about skin cancer and the increasing use of sunscreen may be one of the reasons for the hike in low vitamin D levels.

Diet is an issue too. Very few foods are natural sources of vitamin D, and although a variety of common foods are fortified with D, they provide relatively small amounts.

Some of the better food sources of vitamin D include:

Cod liver oil (1 tablespoon): 1,360 IU

Swordfish (3 ounces, cooked): 566 IU

Salmon (3 ounces, cooked): 447 IU

Tuna fish (3 ounces water-packed): 154 IU

Orange juice, fortified (1 cup): 137 IU

Nonfat milk, fortified (1 cup): 115 to 124 IU

Yogurt, fortified (6 ounces): 80 IU

Sardines (2 pieces, drained): 46 IU

Beef liver (3 ounces, cooked): 42 IU

Egg (1 large): 41 IU

Cereal, fortified (1 cup): 40 IU

Choosing Vitamin D Supplements

Because it's inadvisable to expose skin to excess UV rays and it can be challenging to get enough vitamin D from food. One way to boost vitamin D intake is with supplements.

There are two forms of vitamin D supplements: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). While some experts prefer D3 supplements, there is no strong evidence to believe that one is better than the other when consumed in nutritional doses. At high doses, D2 may be less potent.﻿﻿

Vitamin D supplements come in capsule, gummy, liquid, or chewable tablet forms. Vitamin D is also often found in multivitamins and calcium supplements. Whichever you take, it's important to read labels so that you know how much you're getting.

Most vitamin D supplements provide 400 IU per dose, which is several hundred IU shy of the RDA. But most individuals get some vitamin D naturally from sun exposure and by consuming fortified foods.

It is also important to make sure you're not getting too much vitamin D. The upper limit for children over age 9 and adults is 4,000 IU per day. Too much vitamin D can be harmful. Signs of toxicity include nausea, vomiting, poor appetite, constipation, weakness, and weight loss.﻿﻿

Excess vitamin D also can cause damage to kidneys and increase the levels of calcium in blood—a condition called hypercalcemia that can cause confusion, disorientation, and problems with heart rhythm.

Note, too, that vitamin D supplements can interact with a number of medications, among them steroids, certain cholesterol-lowering drugs, and medications for preventing epileptic seizures.﻿﻿

Given that there are no official guidelines for using vitamin D supplements for preventing or treating thyroid disease, and that taking them can be tricky, it's important to check with your doctor before you add vitamin D to your daily regimen.