Synthroid is a prescription medication commonly used to treat hypothyroidism. And while it can help you manage your condition, it's important to be aware of how Synthroid can interact with common supplements so you know how to get the most out of your medicine.
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Hypothyroidism is when you have an underactive thyroid, per the National Institutes of Diabetes and Digestive and Kidney Diseases (NIDDK). The thyroid is a small gland that produces hormones that dictate how your body uses energy. It creates two types of hormones: triiodothyronine, or T3, and thyroxine, or T4, according to University of Michigan Health. Without enough of them, you can experience symptoms like:
- Weight gain
- Cold intolerance
- Joint and muscle pain
- Dry skin or hair
- Heavy or irregular periods
- Slower heart rate
Hypothyroidism is the most common type of thyroid disorder, affecting nearly five in 100 Americans over the age of 12, particularly people assigned female at birth, per the NIDDK. You're also more likely to have an underactive thyroid if you have Celiac disease, diabetes, rheumatoid arthritis, lupus or a type of anemia caused by a vitamin B12 deficiency.
Hypothyroidism symptoms are often treated with the brand-name drug Synthroid — known generically as levothyroxine — which is a synthetic version of T4, per an August 2021 StatPearls article. But Synthroid can interact with other supplements and medications, so it's important to understand what vitamins should not be taken with thyroid medication.
Here are some common Synthroid interactions to be aware of.
Synthroid can cause side effects like fatigue, nausea, diarrhea, weight loss or skin rash, per the StatPearls article. Consult with your doctor if you experience any of these symptoms.
Taking Synthroid and calcium supplements (or vitamins and antacids that contain calcium) can block your body from absorbing Synthroid and make the medicine less effective at treating your hypothyroidism, according to the Mayo Clinic.
This doesn't mean you shouldn't use calcium supplements, but it does mean you should take Synthroid first and give your body time to absorb it. The Mayo Clinic recommends waiting at least four hours before taking any supplements, calcium or otherwise.
There is also some research to suggest that you should hold off on taking levothyroxine with calcium-rich food. One small study of 10 people with hypothyroidism observed that drinking milk at the same time as taking the medication reduced how much Synthroid the body absorbed, per April 2018 research in Thyroid. However, larger studies are needed to better establish this link.
Calcium-rich foods include:
- Dairy products like milk, yogurt and cheese
- Leafy greens like kale, spinach and collard greens
- Soy products like tofu and soybeans
- Certain nuts and seeds like almonds, chia and sesame seeds
- Certain fish like salmon and sardines
Vitamin D is often taken with calcium to help your body absorb the mineral. There's no evidence to show that vitamin D interferes with Synthroid, per September 2018 research in the Indian Journal of Endocrinology and Metabolism, suggesting that it's OK to take it with your thyroid medication.
Check with your doctor about when and how much vitamin D to take while you're on Synthroid.
Taking Synthroid with food — calcium-rich or not — can impair how well you absorb the medicine, per March 2017 research in Lancet. That's why doctors recommend taking it on an empty stomach at bedtime or in the morning, about 30 minutes to an hour before you eat breakfast.
Like calcium, iron and Synthroid don't mix in quick succession, per the Mayo Clinic. The same general guideline applies here, too: Wait at least four hours after taking Synthroid before swallowing your iron supplement or a vitamin that contains iron.
Aluminum, which can be found in many of the same multivitamins and antacids as calcium and iron, can likewise interfere with absorption, according to the Lancet research, so don't take those supplements at the same time as Synthroid.
Iron and calcium support healthy blood and bones, respectively, per the Office of Dietary Supplements and the Mayo Clinic. That's why you don't want to eliminate them from your diet completely. Talk to your doctor about dietary restrictions and guidelines to make sure you get the nutrients you need while also taking Synthroid safely.
Even though in rare instances an iodine deficiency can cause low thyroid function, this is not typically the case in developed countries like the U.S., where iodine-supplemented foods like salt are the norm and deficiency is uncommon, per the Mayo Clinic. As a result, most people don't need a supplement, so if you're considering taking one, speak with your doctor about it first.
And don't take iodine with your thyroid medication unless your doctor instructs you to. Why? Iodine only treats hypothyroidism if a deficiency is the cause. Otherwise, too much of the supplement can sometimes make hypothyroidism worse, per the Mayo Clinic, so it's best to skip taking iodine and Synthroid together.
But just because the amino acid is related to thyroid function doesn't mean taking the supplement will help with hypothyroidism, per Mount Sinai. In fact, L-tyrosine and levothyroxine can interact, which is why you should avoid the supplement unless your doctor orders it, according to University of Michigan Health.
5. Other Medications
Certain medications can also interact with Synthroid, according to the Cleveland Clinic. Check in with your doctor about your other prescriptions before starting Synthroid to make sure you can take all your medicines safely.
Per the Cleveland Clinic and the StatPearls article, medications that may interact with Synthroid include:
- Certain mood disorder medications like ketamine and lithium
- Proton pump inhibitors like omeprazole and esomeprazole
- Certain medications for high cholesterol like cholestyramine, clofibrate and colestipol
- Certain heart failure medications like digoxin
- Certain diabetes medicines like metformin
- Female hormones like estrogens or progestins in birth control or hormone therapy
- Weight-loss pills like orlistat
Work with your doctor to create a regimen that includes the vitamins, minerals and medications you need to supplement your diet or other underlying health issues while taking Synthroid. For instance, your doctor may recommend you take vitamin B12 along with your thyroid medicine if your hypothyroidism is the result of anemia from B12 deficiency.
Is This an Emergency?
- National Institutes of Diabetes and Digestive and Kidney Diseases: "Hypothyroidism (Underactive Thyroid)"
- University of Michigan Health: "Thyroid Hormone Production and Function"
- StatPearls: "Levothyroxine"
- Mayo Clinic: "Can calcium supplements interfere with hypothyroidism treatment?"
- Thyroid: "Concurrent Milk Ingestion Decreases Absorption of Levothyroxine"
- Lancet: "Hypothyroidism"
- Indian Journal of Endocrinology and Metabolism: "The Effects of Vitamin D Supplementation on Thyroid Function in Hypothyroid Patients: A Randomized, Double-blind, Placebo-controlled Trial"
- Mayo Clinic: "Can iodine supplements help control thyroid function in a person with hypothyroidism?"
- Mount Sinai: "Tyrosine"
- Cleveland Clinic: "Levothyroxine tablets"
- Office of Dietary Supplements: "Iron"
- Mayo Clinic: "Calcium and calcium supplements: Achieving the right balance"