Hypothyroidism, also known as underactive thyroid, is a condition where the thyroid gland does not produce enough thyroid hormones. Thyroid hormones play an important role in regulating metabolism, heart rate, body temperature, and many other bodily functions. When levels of thyroid hormones are low, it can lead to symptoms like fatigue, weight gain, feeling cold, hair loss, and more.
Hypothyroidism is typically treated by replacing the missing thyroid hormones with medication like levothyroxine (Synthroid, Levoxyl). However, some medications can interfere with thyroid hormone replacement and make hypothyroidism worse. It is important for people with hypothyroidism to be aware of medications that should be avoided or used with caution.
Medications That Can Worsen Hypothyroidism
Lithium
Lithium is used to treat bipolar disorder and stabilize mood. Lithium can impact how the thyroid gland functions and the production of thyroid hormones. It has been associated with causing hypothyroidism and goiter (enlarged thyroid). People taking lithium should have their thyroid hormone levels monitored regularly.
If taking lithium is necessary, the dose may need to be adjusted and levothyroxine dosage increased to account for the effects on thyroid. Lithium and levothyroxine should be taken at least 4 hours apart.
Amiodarone
Amiodarone is an anti-arrhythmic medication used to treat irregular heart rhythms. It contains a large amount of iodine and can inhibit thyroid hormone synthesis and release. Up to 20% of people taking amiodarone have been reported to develop hypothyroidism or hyperthyroidism.
Thyroid function tests are recommended every 6 months for people taking amiodarone. The dose of levothyroxine may need to be increased if hypothyroidism occurs. Amiodarone and levothyroxine should be taken at separate times of the day.
Interferon-alpha
Interferon-alpha is an immunotherapy drug used mainly to treat chronic hepatitis C infection. Common side effects of interferon-alpha include thyroiditis and worsening of underlying autoimmune thyroid disease.
People taking interferon-alpha should have periodic thyroid function tests to detect thyroid abnormalities early. Levothyroxine dosage adjustments may be required if hypothyroidism develops.
Anti-seizure medications
Some anti-seizure drugs, such as phenytoin (Dilantin) and carbamazepine (Tegretol), speed up the metabolism of thyroid hormones by the liver. Faster metabolism can lower thyroid hormone levels and require an increased dose of levothyroxine.
Phenytoin, in particular, can reduce total thyroxine levels by up to 40% within days of starting the medication. People taking anti-seizure medications should have thyroid function monitored periodically to avoid hypothyroidism.
Rifampin
Rifampin is an antibiotic used to treat tuberculosis and other bacterial infections. It accelerates the liver’s metabolism of thyroid hormones, especially levothyroxine. Studies show that rifampin can reduce total thyroxine concentrations by up to 60% within one week.
When taking rifampin, the levothyroxine dosage often needs to be increased 2-3 fold to maintain normal thyroid hormone levels. Thyroid tests should be done frequently to ensure adequate treatment.
Cholestyramine
Cholestyramine is a cholesterol-lowering medication that reduces reabsorption of bile acids. It can bind to thyroid hormones in the gut and prevent absorption. One study found that taking cholestyramine caused serum thyroid hormone levels to decrease by more than 50%.
To prevent decreased efficacy, levothyroxine and cholestyramine should be taken 4-6 hours apart. Thyroid lab tests should be monitored to determine if a levothyroxine adjustment is needed.
Proton pump inhibitors
Proton pump inhibitors like omeprazole (Prilosec), esomeprazole (Nexium), and lansoprazole (Prevacid) reduce stomach acid production. Several studies show proton pump inhibitors can modestly lower absorption of levothyroxine.
People taking both proton pump inhibitors and levothyroxine may need an increased dose of levothyroxine. Absorption is best when levothyroxine is taken on an empty stomach and proton pump inhibitors are taken separately.
Calcium carbonate
Calcium carbonate supplements, which are used to prevent osteoporosis, can interfere with levothyroxine absorption when taken at the same time. Data suggests calcium supplements can cause a decrease in serum thyroxine levels.
To avoid an interaction, calcium carbonate and levothyroxine should be ingested at least 4 hours apart. Thyroid lab tests help determine if an adjustment in levothyroxine is needed.
Ferrous sulfate
Ferrous sulfate, used to treat iron deficiency anemia, can bind to levothyroxine in the gastrointestinal tract and reduce its absorption. Studies estimate that ferrous sulfate can decrease levothyroxine absorption by 30-50%.
Taking ferrous sulfate and levothyroxine at least 4 hours apart can help prevent this interaction. Frequent lab testing ensures accurate dosing of levothyroxine.
Sucralfate
Sucralfate is a medication used to treat ulcers. It can bind to levothyroxine in the GI tract and reduce its absorption by up to 60%. Due to its significant impact on thyroid hormone levels, sucralfate should be avoided if possible in people taking levothyroxine.
Soy products
Soy contains compounds called isoflavones that can inhibit thyroid hormone synthesis and absorption. Consuming large amounts of soy may exacerbate hypothyroidism in people taking levothyroxine. Moderate soy intake is likely safe, but patients should consult a doctor.
Glucocorticoids
Oral and inhaled glucocorticoids like prednisone, dexamethasone, and budesonide can suppress serum T3 and T4 levels. Glucocorticoids accelerate the metabolism and excretion of levothyroxine, necessitating a higher replacement dose. Thyroid tests help tailor the levothyroxine dosage.
Other Factors That Impact Thyroid Hormone Replacement
In addition to medications, there are other factors that can influence the absorption and efficacy of levothyroxine:
| Factor | Effect on Levothyroxine |
| Age | Absorption decreases with age |
| Body weight | Higher body weight requires a higher dose |
| Pregnancy | Requirements increase during pregnancy |
| Other medical conditions | Malabsorption disorders impair efficacy |
| Time of ingestion | Should be taken on an empty stomach for best absorption |
Ways to Prevent Problems with Levothyroxine
– Take levothyroxine first thing in the morning on an empty stomach with water. Avoid food and beverages, besides water, for at least 30 minutes afterwards.
– Take any medications or supplements that may interfere at least 4-6 hours apart from levothyroxine.
– Notify your healthcare provider about any new medications you start so they can assess for potential interactions.
– Have thyroid function (TSH level) checked regularly, especially when starting or changing medications.
– Take levothyroxine consistently and do not miss doses. Use reminders if needed.
– Inform your provider about any side effects or symptoms that could suggest a need for dosage adjustment.
– Notify your provider if you are pregnant, breastfeeding, or planning major surgery, as your levothyroxine needs may change.
Foods That May Affect Thyroid Function
Some nutrients in food can impact thyroid function by altering hormone synthesis, absorption, and metabolism. Foods to watch out for include:
Cruciferous vegetables
Cruciferous vegetables like broccoli, cauliflower, cabbage, kale, and brussels sprouts are healthy, but they contain goitrogens that can disrupt thyroid function if consumed in very high amounts. Moderate intake is unlikely to affect those taking levothyroxine.
Soy foods
As mentioned earlier, soy can interfere with thyroxine absorption and synthesis. While small amounts of soy are likely safe, soy supplements or large quantities of soy-based foods should be avoided.
Gluten
Those with celiac disease, gluten sensitivity, or wheat allergy may absorb levothyroxine poorly until their condition is under control. A gluten-free diet supports optimal nutrient absorption.
Fiber
Consuming large amounts of fiber can impair levothyroxine absorption. To prevent this, drink plenty of fluids and separate doses of fiber and levothyroxine.
Fatty foods
Fatty foods can delay gastric emptying and hold up levothyroxine absorption. For best results, avoid eating high-fat meals close to taking your levothyroxine dose.
Conclusion
Hypothyroid patients taking thyroid hormone replacement should be aware of factors that can interfere with efficacy and cause suboptimal treatment. Many medications accelerate levothyroxine metabolism or impair its absorption, necessitating higher replacement doses. By taking levothyroxine properly and avoiding problematic medications when possible, optimal thyroid function can be maintained. Patients should have thyroid levels checked regularly and communicate with healthcare providers about new medications or symptoms. With good mutual understanding and monitoring, hypothyroidism can be managed effectively.