BACKGROUND
Hypothyroidism is quite common. The most common cause is autoimmune thyroid disease, which is the cause of hypothyroidism in about 2% of the population. This occurs when the body develops antibodies that get confused and attack the thyroid. These can block or destroy the thyroid, causing hypothyroidism. There are known risk factors for developing hypothyroidism, such as increasing age and female sex. However, the question of whether there are some factors in the environment, such as food or vitamin deficiencies, that can cause autoimmune thyroid disease remains unanswered.

In some studies, lower vitamin D levels correlate with an increased risk of autoimmune thyroid disease. On the other hand, vitamin D supplementation seems to slightly decrease the chance of developing a combined group of autoimmune diseases. This study was performed to answer the question of whether vitamin D supplementation could decrease the chances of developing hypothyroidism.

THE FULL ARTICLE TITLE
Waterhouse M et al. The effect of vitamin D supplementation on hypothyroidism in the randomized controlled D-Health Trial. Thyroid 2023;33(11):1302-1310; doi: 10.1089/ty.2023.0317. PMID: 37698908.

SUMMARY OF THE STUDY
This study was part of a larger evaluation looking at the effect of Vitamin D on various diseases. The D-Health Trial was a large trial conducted between 2014 and 2020 among >20,000 Australians ages 60 to 84 years. Trial participants received 60,000 international units of cholecalciferol (Vitamin D3) monthly or placebo for up to 5 years. The current study included 17,851 subjects and examined the effect of D3 supplementation on the development of hypothyroidism.

Subjects were considered to have preexisting hypothyroidism if they filled a prescription of levothyroxine within 12 months of randomization. They were considered to develop hypothyroidism if there was a new prescription of levothyroxine after 12 months of randomization.

After an average of 4 years of follow up 293 subjects were considered to have developed hypothyroidism. They tended to be older, female, obese or had poorer selfreported overall health. The incidence of hypothyroidism was slightly lower in women in the vitamin D group, but not significantly so. There was no significant difference in men.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This D-Health trial did not show any significant benefit of vitamin D supplementation on the development of hypothyroidism over 4 years. While this study did have a large sample size, the endpoint of hypothyroidism was determined by what we would call a surrogate marker (getting a prescription for levothyroxine) rather than a true diagnosis with blood tests. Since thyroid hormone levels were not used to determine thyroid status, we cannot know the reasons for patients receiving a prescription for levothyroxine. Neither baseline nor treatment vitamin D levels were measured so we cannot determine whether there could be a different outcome in patients starting with low vitamin D levels or those who had significant increase in levels with treatment. In addition, we don’t know if the results would be different in patients who are at increased risk such as those with autoimmune markers for thyroid disease but normal thyroid tests. Thus, this study does not support the use of high dose vitamin D to prevent hypothyroidism in the general population.

— Marjorie Safran, MD

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