Does Vitamin B-12 Interfere With Synthroid Medicines?

A doctor examines a woman's thyroids in her office.
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Synthroid is the brand name of levothyroxine, a synthetic thyroid hormone used primarily to treat irregular thyroid function, mainly in people with hypothyroidism and those who have had their thyroids removed. Vitamin B-12 has not been shown to interact with Synthroid, but you should still talk to your health care provider before taking it, and there are other medication interactions that you should know about if you take Synthroid.


A variety of circumstances can cause hypothyroidism, however, autoimmune conditions such as Hashimoto's and Graves diseases are the most widely seen causes. These conditions cause inflammation, because your immune system produces antibodies that attack your thyroid gland. In the early stages, your thyroid is usually able to keep up with hormone production, so medication is not immediately needed. However, over time, the destruction these conditions cause results in hypothyroidism, which is generally when your doctor will recommend thyroid replacement hormones.


Your thyroid gland uses the nutrients L-tyrosine and iodine to produce two primary thyroid hormones called thyroxine and triiodothyronine. These hormones control your metabolism. Although triiodothyronine is a more potent thyroid hormone, thyroxine is more readily available and easily converted to triiodothyronine when needed, providing more steady hormone levels. So thyroxine is the preferred hormone used to treat hypothyroidism.


Vitamin B-12 is an essential vitamin that helps your body manufacture red blood cells. Like your thyroid, B-12 helps your body produce energy. Although the relationship between B-12 and the thyroid is not yet well understood, a study published in the May 2008 issue of "The Journal of the Pakistan Medical Association," found that B-12 deficiency is prevalent in hypothyroid patients. The study found that close to 40 percent of individuals with hypothyroidism are B-12 deficient and that B-12 replacement helps relieve hypothyroid symptoms.


Metformin is a medication used to control blood sugar. Synthroid can decrease the effectiveness of Metformin when taken at the same time. You should take these medications at least two hours apart. L-tyrosine is a dietary supplement used to improve performance under stress. You should avoid L-tyrosine because your thyroid uses it to make hormones, so it may increase the effect of Synthroid. Synthroid may interact with cholesterol lowering drugs as well. Tell your doctor which vitamins, herbs or medications you take if he has prescribed Synthroid for you.