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By Melissa Headrick, PharmD
Written as a PharmD candidate at Auburn University School of Pharmacy, Auburn, AL
Thyroid-replacement medications, such as levothyroxine (Synthroid, Levoxyl), are used to treat a variety of medical conditions. The most common is hypothyroidism, but these agents also can be used to suppress pituitary thyroid-stimulating hormone (TSH) and for diagnostic purposes. The optimal dose for hypothyroidism should be the minimum that restores normal serum TSH levels. The average maintenance dosage is 75-125 mcg/d orally, but dosages vary among patients and indications.
Many drug classes and individual drugs can interact with thyroid hormones by several different mechanisms. The most common interactions are with prescription agents, such as rifampin, anticonvulsants, certain antacids, warfarin, and digoxin.
In addition to the prescription agents that are detailed in the Table 3 and Table 3b, over-the-counter agents and herbal products have been reported to interact with thyroid hormones. Ferrous sulfate has been shown to reduce thyroid hormone serum levels and therapeutic response. Patients need to be educated to separate the administration of iron salts from the thyroid hormone by as much time as possible. Soy also has been shown to decrease the effectiveness of thyroid hormones. The mechanism is thought to involve impaired absorption and enterohepatic circulation of thyroid hormones, resulting in significant fecal loss of the drug. Patients should avoid taking soy supplements while taking thyroid medications. Two herbal products, bugleweed and lemon balm, also have been reported to interfere with the action of thyroid hormones and thus should not be used by these patients.
If a patient taking thyroid hormone experiences symptoms of hyperthyroidism (tachycardia, warm skin, nervousness, weight loss, insomnia) or hypothyroidism (facial swelling, cold intolerance, dry skin, weight gain), a drug interaction could be occurring. Some possible management options include changing the dose of thyroid hormone, monitoring TSH levels, and separating the interacting agents. Clinical judgment should be used to evaluate these and other drugs for their potential to interact with thyroid hormones.
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