From the Research
You should hold vitamin B12 supplements before getting thyroid levels checked because B12 can potentially interfere with accurate thyroid test results, as suggested by the most recent and highest quality study available 1. The interaction between vitamin B12 and thyroid function tests is complex, and high doses of vitamin B12 might affect the measurement of thyroid stimulating hormone (TSH) and other thyroid hormones in some laboratory tests. Some key points to consider include:
- The prevalence of B12 deficiency in patients with autoimmune thyroid disease is higher than in the general population, as shown in a study from 2006 2.
- Thyroid dysfunction and anemia often co-occur, and current guidelines recommend assessing thyroid function in the work-up of anemia, as discussed in a 2016 study 3.
- Pernicious anemia, a common cause of vitamin B12 deficiency, can have subtle and insidious onset, and diagnostic challenges remain for many practicing clinicians, as highlighted in a 2021 review 4.
- The diagnosis of vitamin B12 deficiency can be complex, and various tests, including serum B12, holotranscobalamin, serum methylmalonic acid, and plasma homocysteine measurements, have their own limitations and pitfalls, as discussed in a 2021 critical review 1. It's generally recommended to stop taking B12 supplements for 24-48 hours before your blood draw, though you should follow your doctor's specific instructions. This temporary pause helps ensure that your thyroid function tests reflect your true thyroid status without any external influences. Always inform your healthcare provider about all supplements you're taking, as they may provide personalized guidance based on your specific situation and the particular laboratory tests being used.