Most patients being treated for hypothyroidism are prescribed levothyroxine, also known as Synthroid, its brand name. In fact, nearly 12 percent of women are on the drug for life. Most commonly caused by autoimmune thyroiditis, hypothyroidism is defined by thyroid underactivity.
Thyroid Drug Interactions
If you aren’t feeling optimal despite receiving thyroid replacement, you may be experiencing the effects of thyroid drug interactions. In some cases, the cause of thyroid medication not working is reduced absorption because of these interactions.
All patients being treated for hypothyroidism should receive regular lab tests to monitor TSH, or thyroid stimulating hormone. If TSH levels fluctuate wildly, getting thyroid medication levels just right can be more difficult.
Thyroid drug interactions can result from use of drugs such as lithium, amiodarone and iodine. However, certain interactions may be challenging to pinpoint. Reviewing the list of drugs that can result in thyroid medication not working may help you identify potential issues with your treatment.
Drugs That Require Raising the Dosage of Levothyroxine
Four drugs, in particular, can make thyroid replacement less effective. If this happens, your current dose of Synthroid or levothyroxine may not be enough. These drugs are:
- Proton pump inhibitors, or PPIs: Used to treat acid reflux, this class of drugs includes lansoprazole, or Prevacid, omeprazole, or Prilosec, and pantoprazole, or Protonix. By inhibiting stomach acid production, these medications prevent levothyroxine from being absorbed properly.
- Estrogens: Whether taken for hormone replacement therapy or oral contraception, estrogen can reduce the amount of free thyroid hormone circulating in the body. This happens because of increased production of thyroid binding globulin. When patients take estrogen, they commonly need a higher dosage of levothyroxine.
- Iron: Iron supplements bind thyroxine, leading to reduced absorption and making less thyroxine available for the body to use.
- Calcium: Supplemental calcium, most often calcium carbonate, may be taken for prevention of acid reflux or osteoporosis. Research has shown that levels of free thyroid hormone are reduced in patients who take these supplements with their levothyroxine. The calcium and levothyroxine bind together, resulting in lower levels of bioavailable thyroxine.
Drugs That Necessitate Reduction of Levothyroxine Dosage
Two other drugs may result in thyroid medication not working properly by increasing their effects. In this situation, the dose of levothyroxine must be adjusted downward. These drugs are:
- Metformin: In some studies, the diabetes drug metformin has been shown to reduce TSH. Although this may indicate that receptor affinity of thyroid hormones would be affected, we do not yet know for sure.
- Statins: Drugs such as Crestor, Zocor and Lipitor are prescribed to lower cholesterol levels. One study examining 11,000 patients taking levothyroxine found that their TSH decreased when they used statins, necessitating reductions of levothyroxine dosage. At present, we do not know whether thyroid drug interactions with statins are related to changes in absorption of thyroxine in the gastrointestinal tract or whether liver metabolism is altered. Because these drugs are commonly used together, patients should be vigilant for this interaction.
In one recent clinical trial, researchers found that TSH of patients receiving levothyroxine was not affected by H2 receptor antagonists, such as Zantac, Tagamet and Pepcid, or by glucocorticoids, also called steroids.
The seriousness of thyroid medication not working makes watching for interactions essential. In some cases, patients may benefit from switching to another medication to avoid thyroid drug interactions with the guidance of their doctor.
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