Can People on Synthroid Take Diet Pills?

Male doctor holding pills
A doctor is holding pills in his hand. (Image: DeanDrobot/iStock/Getty Images)

Synthroid, also known by its generic name levothyroxine, is taken for hypothyroidism -- a low-functioning thyroid gland. It's a hormone replacement for people who don't make enough thyroid hormone -- the hormone responsible for regulating metabolism. Hypothyroidism often leads to weight gain, although once you're on the correct dosage of Synthroid, your metabolism should normalize and you should be able to lose weight. Diet pills are not recommended because they can interfere with the absorption of your thyroid hormone replacement.

Hypothyroidism

Hypothyroidism is most often caused by an inflammation of the thyroid gland, which could be the result of an autoimmune disorder. Other causes of a sluggish thyroid include birth defects, certain drugs and removal or radiation of the thyroid as part of the treatment for hyperthyroid -- an overactive thyroid gland. When your thyroid doesn't produce enough T3 and T4 hormones your metabolism -- the rate at which your body burns calories -- slows down. Not only can this lead to weight gain, but you may also feel fatigued, weak, depressed and be more sensitive to cold. Taking thyroid hormone replacement drugs, probably for the rest of your life, may be the only way to regulate your hormones and metabolism.

Alli and Synthroid

Synthroid is designed to be absorbed in your intestines and should be taken on an empty stomach at least one hour before or two hours after eating. Any substance that disrupts or changes digestion can stop your body from fully absorbing the replacement thyroid hormone. Alli, an FDA-approved weight-loss pill, works by blocking lipase, the enzyme that digests fat, and allowing fat to pass undigested, thereby reducing your calorie intake. Drugs.com says that orlistat, the active ingredient in Alli, interferes "with the gastrointestinal absorption of levothyroxine and other thyroid hormones." However, it may be possible to use orlistat if it is taken four hours after your Synthroid medication.

Metformin

Both Glucophage and Byetta are diabetes medications that are being prescribed for weight-loss. Glucophage increases insulin sensitivity, helping you to consume fewer calories because you aren't as hungry. Byetta slows digestion, holding food in your stomach for a longer time, which increases fullness -- also leading to a reduction in caloric intake and weight loss. Thyroid hormones affect Glucophage and can lead to high blood sugar levels, called hyperglycemia and can make diabetes worse, states Drugs.com.

Timing

Once Synthroid is absorbed, it may be possible to take other diet aids. Pharmacist Dr. Kristi Monson notes that waiting four hours after taking your Synthroid seems to avoid most drug interactions. Certain minerals such as calcium and iron interfere with your body's ability to assimilate thyroid hormone and should also be taken at least four hours after your Synthroid. Diabetes medication dosages may need to be adjusted; it's also possible to increase the doses of certain medications to compensate for the interaction.

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