Published on July 21, 2021

Study shows optimal thyroid function may require a vitamin D level of at least 50 ng/ml (125 nmol/L)

The thyroid is a gland that produces, stores, and releases thyroid hormones, which are important to the function of many systems and organs, including the heart, brain, liver, kidneys and skin. Thyroid hormones help regulate vital functions, such as breathing, heart rate, muscle strength, menstrual cycles, body temperature, metabolism, and more.

The thyroid is controlled through the hypothalamus-pituitary-thyroid axis, which is prone to circadian and seasonal changes. Previous studies have demonstrated an association between lower thyroid function during the winter months (when vitamin D levels are at their lowest) compared to the summer months (when people tend to have higher vitamin D levels), as well as an increased risk of autoimmune thyroid disease during the winter, especially among individuals with lower vitamin D levels.

The Thyroid May Need Higher Vitamin D Levels to Function at its Best

Vitamin D has many functions within the body, with response curves varying by system (such as skeletal vs immune). For example, while bone health may benefit from a vitamin D level of only 20 ng/ml (50 nmol/L), the immune system may require a much higher level of at least 50 ng/ml (125 nmol/L). Research, such as that discussed below, supports a higher level of vitamin D for thyroid function, similar to what may be needed for optimal immune system function.

A study by Mirhosseini it al., and including GrassrootsHealth Research Director Samantha Kimball, analyzed data from a group of 11,017 wellness program participants to determine if there was a relationship between vitamin D and thyroid function. Vitamin D and thyroid measures (including TSH, FT4, FT3, and thyroid antibodies) were taken at baseline and at follow-up, about one year later. Vitamin D supplementation was given to all participants (average dose 6000 IU/day) with the goal of achieving a vitamin D level of at least 40 ng/ml (100 nmol/L).

Those with Low Thyroid Function were Much More Likely to be Vitamin D Deficient

In this study, similar to previous studies, individuals presenting with low thyroid function (hypothyroid) were three times more likely to be vitamin D deficient (<20 ng/ml or <50 nmol/L) than those with healthy thyroid function, and those presenting with subclinical hypothyroidism were about twice as likely to be vitamin D deficient. Also, baseline TSH levels were higher and FT4 levels significantly lower (both indicating below optimal thyroid function) in the winter than in the summer among those who did not supplement with any vitamin D.

Thyroid Function Improved with Vitamin D Supplementation Program

The dose of vitamin D supplementation for each participant was determined using a calculation similar to that used with the GrassrootsHealth vitamin D*calculator, using the baseline vitamin D level and a target of at least 40 ng/ml (100 nmol/L). In this study, vitamin D supplementation to target a level of at least 40 ng/ml resulted in a significant reduction of TSH and the detection of hypothyroidism and subclinical hypothyroidism.

At the beginning of the study, excluding those on thyroid medications, 1.3% of the participants presented with blood work characteristic of hypothyroidism. At the end of the study, only 0.3% of participants were hypothyroid. 21.7% of participants showed characteristics of subclinical hypothyroidism at baseline, compared to 6.1% at follow up. There was also a significantly lower risk of reporting brain fog, low mood, unrefreshing sleep, weight gain, or low energy (symptoms of low thyroid function) at the end of the study.

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Overall, this study found a 30% reduced risk of hypothyroidism, and a 32% reduced risk of elevated anti-thyroid antibodies (covered in more detail in an upcoming post) among participants whose vitamin D level was 50 ng/ml (125 nmol/L) or higher.

Vitamin D levels below 50 ng/ml (125 nmol/L) were also related to

  • an overall increased risk of low thyroid function and thyroid disease
  • a 107% increased risk of elevated TSH
  • a 60% and 14% lower risk of low FT4 and FT3

Of note, vitamin B12 status was also significantly associated with low thyroid function, and improving vitamin B12 status was shown to significantly increase thyroid hormones FT3 and FT4.

In conclusion, this study suggests that vitamin D may influence thyroid function, and supplementation to achieve a vitamin D level between 40-60 ng/ml (100-150 nmol/L) may be useful in helping to prevent hypothyroidism and autoimmune thyroid disease.

Make Sure You Are Getting Enough Vitamin D and Other Important Nutrients – Measure Your TSH as Well!

Having and maintaining healthy vitamin D levels and other nutrient levels can help improve your health now and for your future. Choose which to measure, such as your vitamin D, omega-3s, and essential minerals including magnesium and zinc, by creating your custom home test kit today. Take steps to improve the status of each of these measurements to benefit your overall health. You can also track your own intakes, symptoms and results to see what works best for YOU.

Enroll and test your levels today, learn what steps to take to improve your status of vitamin D (see below) and other nutrients and blood markers, and take action! By enrolling in the GrassrootsHealth projects, you are not only contributing valuable information to everyone, you are also gaining knowledge about how you could improve your own health through measuring and tracking your nutrient status, and educating yourself on how to improve it.

Help everyone Move Research into Practice with vitamin D and other nutrients! As a special birthday gift to everyone, in honor of the science, we have created a special scholarship fund for anyone to donate to that will go towards helping others participate. Your donation will allow anyone to get help with funding their participation when they need it.

Text-to-give: Text Daction to 44321 to add to our Scholarship Fund.

What Does it Take YOU to Get Your D to 40 ng/ml (100 nmol/L)?

Did you know your health could be greatly affected by making sure you have a vitamin D level of at least 40 ng/ml (100 nmol/L)? Help us help you.

STEP 1 - Do you know what your vitamin D level is? If not, be sure to test today to find out.

STEP 2 – Determine your target level. Are you at your target level? Experts recommend a level of at least 40-60 ng/ml (100-150 nmol/L).

STEP 3 – Need to boost your level? Use the D*calculator to see how much vitamin D it may take to reach your target. Opt for the Loading Dose for a quicker boost.

STEP 4 – Optimize how your body absorbs and utilizes vitamin D with co-nutrients and these simple steps.

STEP 5 – Re-Test! This is an important step to make sure you have reached your target level, and to ensure you are not taking too much! Re-testing after 3-4 months is recommended.

STEP 6 – Adjust, Repeat…

Give your immune system the nutrients it needs to support a healthy you and protect yourself from unnecessary diseases, especially COVID-19.


The first Randomized Controlled Trial on vitamin D and COVID-19 has shown a 96% lower risk of ICU admission for those receiving vitamin D (as 25(OH)D to quickly boost vitamin D blood levels) along with the standard treatment, compared to those receiving standard treatment alone.

These results support many previous observational studies showing a relationship between vitamin D levels and intake and COVID-19 severity.

Review the Latest Nutrient Research for COVID-19

GrassrootsHealth Nutrient Research Institute has launched the new Immune Boost project with the use of our myData-myAnswers nutrient health system that nearly 15,000 people are already using for their health. Specific markers that influence immune health are suggested for testing as part of this project including:

  • Vitamin D
  • Omega-3 Index
  • Essential elements magnesium, selenium, and zinc
  • hsCRP

Our goal is to demonstrate how one can use the Nutrient Research Model established by Dr. Robert Heaney to show the effect of vitamin D serum levels of at least 40 ng/ml (100 nmol/L) on risk reduction for all ethnicities in the population. Status and intake of other nutrients will also be analyzed for any type of relationship to immune status and symptom severity. Join the project today!

Please let us know if you're interested in helping sponsor this project.

CLICK HERE for updates and new information about the project.

Through GrassrootsHealth Nutrient Research Institute, you can also test your essential elements magnesium, copper, zinc and selenium, toxins such as lead, mercury and cadmium, as well as your omega-3 levels, inflammation levels and thyroid stimulating hormone (TSH) level. Find out your levels today! Log on to the test selection page (click the link below) to get your tests and see for yourself if your levels can be improved.

Make sure you track your results before and after, about every 6 months!

Click Here to Access the Test Page

How can I track my nutrient intake and levels over time?

To help you track your supplement use and nutrient levels, GrassrootsHealth has created the Personal Health Nutrient Decision System called

For each specific supplement, you can track what days you take it, how much, and many other details.  This will help you know your true supplemental intake and what patterns of use work for you to reach and maintain optimum nutrient levels. Check it out today!