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Can You Overdose On Vitamin B12 With A Thyroid Condition?

Find out if it's possible to get too much vitamin B12 from your vitamin supplements.
Can You Overdose On Vitamin B12 With A Thyroid Condition?
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Vitamin B12 is essential for forming red blood cells, nerve function, producing DNA, and cellular metabolism. As you can imagine, a B12 deficiency can lead to numerous problems, particularly anemia. So, having optimal levels of this vital nutrient is critical for our overall health. But can you get too much of a good thing when it comes to this vitamin? Well, the good news is most sources say it is pretty difficult to overdose on vitamin B12, but too much of it may cause you to have a few unpleasant symptoms. Let's take a look. 


What is vitamin B12?


Also called cobalamin, vitamin B12 is a nutrient we must get from food sources to increase the levels in our body. The most common natural sources of vitamin B12 come from animals, such as poultry, meat, fish, and dairy. However, you may also see vitamin B12 fortified in some foods like cereals. 


Most people in the United States do not have a true B12 deficiency. However, certain groups may be at greater risk for this problem, including:

  • Vegetarians
  • Vegans
  • Older adults
  • People with digestive problems


Signs of a B12 deficiency include fatigue, anemia, muscle weakness, nerve problems, mood disturbance, and intestinal problems.


How is vitamin B12 absorbed?


A two-step process is required for your body to absorb vitamin B12 fully. First, the hydrochloric acid in your stomach helps separate the vitamin from the protein attached to it when you ingest it. Secondly, the freed-up vitamin B12 must combine with another protein (called the intrinsic factor) in the stomach so that the body can absorb it.


Without intrinsic factor, people cannot absorb vitamin B12 in their gut. Individuals who lack intrinsic factor (IF) are especially prone to pernicious anemia and neurological problems. Some babies are born without intrinsic factor, and people with certain autoimmune disorders may no longer have IF if inflammation causes their stomach lining to atrophy.


Who needs supplemental B12?


Ideally, most people will get enough B12 from a balanced diet, especially in infancy through middle adulthood. However, older adults may have a more challenging time absorbing B12 from their diet, so it may be necessary to supplement this vitamin. Also, people with digestive problems, including those with inflammatory bowel disease, Crohn's, celiac disease, and other autoimmune medical conditions, may need to take additional B12. Sometimes, the immune system mistakenly attacks the cells in the stomach lining that produce IF.


Common symptoms of a vitamin B12 deficiency include:

  • Fatigue
  • Shortness of breath
  • Dizziness
  • Unsteady movements
  • Muscle weakness
  • Irregular heartbeats
  • Weight loss
  • Numbness in the hands and feet
  • Confusion and forgetfulness
  • Pale or yellowed skin


Because some people lack IF, getting vitamin B12 another way aside from through diet is necessary. Some people can take a B12 tablet. However, this may not be the best route if you lack intrinsic factor. So, some other ways you can get B12 include: 

  • Taking sublingual tablets
  • Intramuscular injections
  • Intranasal gel


Most people prefer sublingual tablets because they are more affordable and do not require regular injections. However, you will want to consult your doctor to see which route is best for you, along with the best dose. 


What is the recommended amount of B12 for people with thyroid conditions?


Not all people with a thyroid condition will need to take supplemental B12. However, if you have autoimmune thyroid disease like Hashimoto's disease (the most common cause of hypothyroidism or underactive thyroid), you may have more difficulty absorbing B12 in your gut. Some studies show that people with TPO antibodies are prone to lower levels of vitamin B12. In this case, you will want to make sure your healthcare provider orders blood work for B12 periodically. 


Also, many people with one autoimmune disorder often have multiple conditions. For example, people with Hashimoto's disease are likely also to have celiac disease or rheumatoid arthritis. 


The Recommended Daily Intake of B12 is 2.4mcg. However, you will find that most vitamin supplements contain much higher doses of vitamin B12, such as 500mcg or 1000mcg. According to the NIH, these doses are safe as vitamin B12 is a water-soluble vitamin of which your body will only absorb what it needs and excrete the rest. 


If you need to boost your vitamin B12 intake, your doctor should advise you on the right amount based on what your bloodwork shows. Interestingly, your body stores 1,000 to 2,000 times the amount of vitamin B12 you need to function, so it can take years for a deficiency to show up in your bloodwork and with symptoms. However, because many people live undiagnosed for years with autoimmune conditions like celiac disease or Hashimoto's (and often both), it can often show up around the same time. 

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What does a B12 overdose look like?


There has been no Upper Tolerable Intake Level established for B12 supplementation. Per the NIH, "B12 has not been shown to cause any harm, even at high doses." As noted above, it is difficult to overdose on B12. Indeed, your body already stores more B12 than you need to function each day. 


With that said, taking too much oral, intranasal, or intramuscular B12 may cause some of the following symptoms:

  • Headaches
  • Nausea and vomiting
  • Diarrhea
  • Tingling in the hands and feet
  • Fatigue or weakness


You should be aware of medication interactions with B12, such as metformin and proton pump inhibitors (PPIs). If you are taking other medications, talk with your doctor first before adding B12 to your regimen.


If you have Hashimoto's thyroiditis, you may be at greater risk for vitamin deficiencies, so it is essential to have a complete workup and regular blood tests periodically. Paloma Health's thyroid doctors are well aware of the importance of checking for nutritional deficiencies in thyroid patients, so schedule a consult today to ensure you are optimizing your thyroid health. 


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Julia Walker, RN, BSN

Clinical Nurse

Julia Walker, RN, BSN, is a clinical nurse specializing in helping patients with thyroid disorders. She holds a Bachelor of Science in Nursing from Regis University in Denver and a Bachelor of Arts in the History of Medicine from the University of Colorado-Boulder. She believes managing chronic illnesses requires a balance of medical interventions and lifestyle adjustments. Her background includes caring for patients in women’s health, critical care, pediatrics, allergy, and immunology.

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