B vitamins and the thyroid: what B12 has to do with Hashimoto’s

September 28, 2026approx. 15 minutesby mybody®x Editorial & Specialist Team

In this article

Thyroid: What are an underactive and an overactive thyroid?

With an underactive thyroid, the thyroid gland does not produce enough hormones; with an overactive thyroid, it produces more than needed. In countries such as Germany, approximately 5 in 100 people have an underactive thyroid (IQWiG, 2024).

Thyroid hormones affect metabolism. When they are lacking, it slows down, which can cause a wide range of symptoms, such as fatigue or constipation, according to the Institute for Quality and Efficiency in Health Care (IQWiG, 2024).

With an overactive thyroid, it is the opposite. When too many hormones are produced, many bodily functions unnecessarily “go into overdrive,” according to the IQWiG (IQWiG, 2024). An overactive thyroid affects approximately 1 in 100 people and is more common in women than in men (IQWiG, 2024).

Hashimoto’s thyroiditis, or Hashimoto’s for short, is inflammation of the thyroid caused by the immune system. Over time, it often leads to an underactive thyroid, and in Germany, Hashimoto’s is the most common cause (IQWiG, 2024). An estimated 4 in 1,000 women and 1 in 1,000 men develop it, most between the ages of 30 and 50 (IQWiG, 2024).

Thyroid disorders in numbers

Source: Institute for Quality and Efficiency in Health Care (IQWiG), gesundheitsinformation.de, 2024

By this calculation, the rate among women is around four times that among men (IQWiG, 2024). B12 is one of the B vitamins and works closely with folate, another B vitamin, in metabolism, making it important for cell division and blood formation (DGE, 2025). If you live with a thyroid diagnosis, you might wonder whether your thyroid needs more of it. The honest answer depends on the condition, and in Hashimoto’s, the answer leads through the stomach.

The article Vitamin D3 Deficiency and the Thyroid explains how vitamin D and the thyroid are connected: vitamin D there, B12 here, and the question of what about the other B vitamins.

Hashimoto’s and vitamin B12: What does the stomach have to do with it?

With Hashimoto’s, the path to vitamin B12 deficiency can lead through the stomach: stomach disorders have been reported in around 10 to 40 percent of people with Hashimoto’s (Cellini et al., 2017).

Conversely, around 40 percent of people with autoimmune gastritis, an inflammation of the stomach triggered by the immune system, also have Hashimoto’s (Cellini et al., 2017). This connection between the thyroid and stomach, known as thyrogastric syndrome, was first described in the early 1960s (Cellini et al., 2017).

The IQWiG health information service adds that Hashimoto’s thyroiditis can occur alongside other autoimmune diseases (IQWiG, 2024). Studies on gastritis describe which ones affect the stomach and what this means for B12.

How does the body absorb B12 through the stomach and intestines?

For B12 from food to reach the intestinal cells, it must be bound to intrinsic factor, a substance produced in the stomach cells (DGE, 2025). Stomach diseases can affect the production of this factor and then lead to B12 deficiency regardless of intake (DGE, 2025). In this case, the problem is not the amount on the plate, but absorption.

This is exactly where autoimmune gastritis comes in. The UK National Institute for Health and Care Excellence describes it as inflammation of the body of the stomach, which usually causes no symptoms itself but can destroy the parietal cells (NICE, 2024). Parietal cells are cells in the stomach lining that produce stomach acid and intrinsic factor (Cellini et al., 2017).

When they are lost, a double gap arises: Less stomach acid is produced to release B12 from food, and less intrinsic factor is produced to enable absorption (NICE, 2024). The same lack of acid also reduces iron absorption (NICE, 2024). You can think of parietal cells as a post office: The acid opens the envelope, and intrinsic factor writes the address the intestines need to accept the package. If the post office stops working, less gets through, even when there is plenty of mail.

How can inflammation of the stomach lead to anemia?

Cellini and colleagues describe a sequence: First, the lack of acid can cause iron-deficiency anemia; later, as the stomach lining continues to shrink, pernicious anemia can develop (Cellini et al., 2017). This refers to the anemia at the end of this chain, when parietal cells and intrinsic factor are absent. Its life-threatening form is very rare today because B12 deficiency is recognized and treated more effectively (NICE, 2024).

Absorption of levothyroxine, the active ingredient in thyroid hormone tablets, can also be impaired in autoimmune gastritis (Cellini et al., 2017). Anyone taking these tablets should discuss this with their healthcare provider.

A study from Israel involving 115 people with autoimmune thyroid disease shows what this can look like in a clinical setting: 32 of them, or 28 percent, had low B12 levels (Ness-Abramof et al., 2006). The authors classified some of these cases as pernicious anemia, identified by elevated gastrin alongside low B12; gastrin is a lab value they used to assess the stomach (Ness-Abramof et al., 2006). All five participants with elevated gastrin who underwent a gastroscopy with a tissue biopsy were found to have atrophic gastritis, an inflammation of the stomach involving thinning of the lining (Ness-Abramof et al., 2006).

The authors also note that signs of B12 deficiency can be subtle and go unnoticed, especially in people with a known autoimmune disease (Ness-Abramof et al., 2006). The article Anemia caused by B12 and folate deficiency explains how B12 deficiency leads to anemia and how it differs from iron deficiency.

How common is B12 deficiency in thyroid disease?

It depends on the condition. A 2023 meta-analysis found B12 deficiency in 18 percent of people with autoimmune thyroid disease and 6 percent of people with hyperthyroidism (Benites-Zapata et al., 2023).

A meta-analysis statistically combines the results of individual studies. The study by Benites-Zapata and colleagues analyzed 64 studies with a total of 28,597 participants (Benites-Zapata et al., 2023). Autoimmune thyroid disease is an umbrella term for thyroid diseases caused by the immune system, including Hashimoto’s.

The table shows the pooled proportions, meaning those combined across all studies, of people with B12 deficiency, alongside how the mean B12 level differed from that of healthy control participants.

B12 in thyroid disease · 2023 meta-analysis

A difference was found for hypothyroidism: B12 levels averaged 60.67 picograms per milliliter (pg/ml) lower than in healthy people (Benites-Zapata et al., 2023). By contrast, the analysis found no significant difference in mean B12 levels for autoimmune thyroid disease or hyperthyroidism (Benites-Zapata et al., 2023).

What does this mean for Hashimoto’s and B12?

The figures appear in the study’s summary without a comparison value for people without thyroid disease. It is therefore not possible to tell from this whether people with Hashimoto’s are more likely to have a B12 deficiency than healthy people, especially since the average levels did not differ significantly (Benites-Zapata et al., 2023). These are also observational data: they show how common a low level was in a group, not what caused it.

Individual studies vary widely. A review found proportions ranging from about 6 to 55 percent in three studies of autoimmune thyroid diseases (Collins and Pawlak, 2016). Of 7,091 studies screened, only 6 were included in the analysis, and according to the authors, the wide range also reflects the nutritional status of the populations studied (Collins and Pawlak, 2016).

The proportion was 28 percent in the Israeli clinic group, in a small study conducted at a single clinic (Ness-Abramof et al., 2006). Of the 115 participants, 108 were women (Ness-Abramof et al., 2006); this describes the makeup of the group, not a greater likelihood of women being affected.

Vitamin B12 and hyperthyroidism: What changes?

For B12 levels, the meta-analysis found no significant difference compared with healthy people; 6 percent of people with hyperthyroidism had a deficiency (Benites-Zapata et al., 2023).

IQWiG describes in detail how hyperthyroidism changes the body. When there are more thyroid hormones in the blood than needed, various metabolic processes and bodily functions become unbalanced, which can be noticeable through symptoms such as a racing heart or weight loss despite increased appetite (IQWiG, 2024).

The most common cause is Graves’ disease: an immune system malfunction causes the thyroid to produce more hormones (IQWiG, 2024). Hashimoto’s thyroiditis can also rarely involve a brief period of hyperthyroidism at the beginning (IQWiG, 2024). Hyperthyroidism usually occurs in middle adulthood (IQWiG, 2024).

Does hyperthyroidism increase the need for B vitamins?

It’s a reasonable thought: if everything is running faster, perhaps the body also needs more vitamins. But this hasn’t been established. IQWiG’s health information describes an accelerated metabolism with an overactive thyroid (IQWiG, 2024), but says nothing about vitamin requirements.

The meta-analysis also found no significant difference in average B12 levels between people with an overactive thyroid and healthy people (Benites-Zapata et al., 2023). The sources reviewed here do not establish an increased need for B12 or other B vitamins with an overactive thyroid.

So, if you’re looking for the right vitamins for an overactive thyroid, there’s no simple list—just some context. Testing shows whether you’re deficient in a nutrient, and a healthcare professional can advise you on what to do based on the results.

Do you need more B vitamins for your thyroid?

Not necessarily. Hashimoto’s can lead to B12 deficiency through stomach problems, but average B12 levels did not differ significantly between people with autoimmune thyroid disease and healthy people (Benites-Zapata et al., 2023).

Two claims come up around the thyroid and vitamins. Both go further than the evidence supports.

Claim and evidence

That’s a fair question: If a deficiency is possible with Hashimoto’s, why not simply take precautions? Because a possible deficiency is something to check with a test. Statistics can’t tell you whether you’re deficient, and a healthcare professional can advise you on what to do based on the results.

Diet is also part of the picture. If you live with an underactive thyroid, you can find more context in the article Diet for an Underactive Thyroid. As for B12, the German Nutrition Society notes that stomach diseases can cause a deficiency regardless of intake (DGE, 2025).

Which blood tests show thyroid function and B12 status?

For thyroid function, TSH and the thyroid hormones themselves; to check for Hashimoto’s, antibodies against thyroid tissue (IQWiG, 2024). The B12 level in your blood offers an initial indication of your B12 status.

IQWiG writes: “Because many symptoms of an underactive thyroid can also have other causes, blood tests are performed to make a clear diagnosis.” (IQWiG, 2024) The so-called thyroid levels are measured: the thyroid hormones themselves and thyroid-stimulating hormone, or TSH, which is produced in the pituitary gland and regulates thyroid function (IQWiG, 2024).

The thyroid hormones are measured as free triiodothyronine, or fT3, and free thyroxine, or fT4. The article fT3 and fT4 alongside TSH explains what these two values show in addition to TSH. The article Normal TSH level but still experiencing symptoms discusses TSH levels and persistent symptoms.

A further blood test can detect certain antibodies against thyroid tissue and show whether Hashimoto’s thyroiditis is behind impaired function (IQWiG, 2024). In cases of an overactive thyroid, blood tests can also indicate Graves’ disease (IQWiG, 2024). In both cases, the diagnosis is made by a medical practice.

For B12, the blood value is a starting point, not a diagnosis. Read about what a low B12 value may mean and how it is interpreted in the article Vitamin B12 deficiency.

What does the Women’s Wellness Check measure for thyroid function and B12?

You can also have some of these values measured at home. The Women’s Wellness Check by mybody®x (MYBODY Lab GmbH) measures TSH and the two free thyroid hormones fT3 and fT4, along with vitamin B12, with analysis performed by a specialist laboratory in Germany.

What it can't do is part of the picture: it doesn't measure thyroid antibodies, so it can't diagnose Hashimoto’s disease. It also doesn't include hemoglobin, the red blood pigment measured in a blood count that a medical practice uses to detect anemia. The test provides a snapshot to discuss with your medical practice.

Limitations: What a blood test for B12 and thyroid function cannot determine

A blood test provides a snapshot, not a diagnosis. It shows what a level was on the day the sample was taken, but not why.

A low B12 level can be caused by the stomach in people with Hashimoto’s. But there can be other reasons too: intestinal conditions, for example, can reduce B12 absorption (DGE, 2025). Your healthcare practice can determine the cause.

The study figures are observational data from groups. They do not indicate whether an individual with Hashimoto’s has a deficiency, and according to the authors, the range across individual studies also depends on the nutritional status of the populations studied (Collins and Pawlak, 2016).

The Women’s Wellness Check does not measure thyroid antibodies or hemoglobin. It therefore cannot confirm or rule out Hashimoto’s thyroiditis, and the practice diagnoses anemia through a blood count.

If you have symptoms or take thyroid medication, your primary care practice is the first place to go. They can assess your thyroid levels and B12 together and decide whether further tests are needed.

Hashimoto’s and B12: Which question can help you in your doctor’s appointment?

The question is not whether you need more vitamins, but whether your stomach is involved. Whether you have a B12 deficiency with Hashimoto’s and what is causing it can be measured and investigated.

Two questions can move the conversation forward. First: Was B12 tested when your thyroid condition was diagnosed? Second: Could a low level point to stomach involvement that should be investigated further? Both questions focus on the cause rather than just the deficiency, and both are easier to ask when you have test results in hand.

With an overactive thyroid, the honest answer is shorter: metabolism speeds up, but an increased need for B vitamins has not been proven. The question points to the thyroid. The answer may lie in the stomach.

Frequently asked questions

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Sources

1. Benites-Zapata et al.: Vitamin B12 in thyroid disorders, meta-analysis, Frontiers in Endocrinology (as of 2023) – europepmc.org
2. Cellini et al.: Hashimoto’s thyroiditis and autoimmune gastritis, Frontiers in Endocrinology (as of 2017) – europepmc.org
3. IQWiG: Hashimoto’s thyroiditis and hyperthyroidism, gesundheitsinformation.de (as of 2024) – Hashimoto’s · Hyperthyroidism
4. Collins and Pawlak: B12 deficiency in thyroid disorders, Asia Pacific Journal of Clinical Nutrition (as of 2016) – europepmc.org

The proportions and average values for B12 deficiency are based on [1], and the range from individual studies on [4]. The pathway through the stomach is described in [2], as well as in NICE guideline NG239 (2024) and the DGE’s B12 questions and answers (2025). Information on Hashimoto’s and hyperthyroidism comes from [3], supplemented by the IQWiG page on hypothyroidism (2024); the clinical study from Israel is Ness-Abramof et al. (2006). Information on the Women’s Wellness Check comes from mybody®x product data; the price was checked on 28.09.2026.