Xenical's effect on an underactive thyroid

Updated November 21, 2016

Xenical is a prescription diet medication. Its chemical name is orlistat. Described as a lipase inhibitor, Xenical acts to prevent the body's fat-dissolving enzymes from working, which ultimately interrupts fat absorption through the intestine. Xenical has a range of side effects and contraindications, not the least of which is possible problems for people who have underactive thyroids.


According to the website Netdoctor, Xenical has been developed to act on two key enzymes of the digestive system: gastric and pancreatic lipases. These lipases break down fat molecules in the digestive tract that would otherwise be too large to pass as nutrients from the intestinal wall into the bloodstream. Xenical prevents these lipases from acting on the fat molecules, which remain whole and exit the body as waste.


Xenical is prescribed specifically for obese and overweight individuals. The manufacturer recommends its use for people who have a body mass index (BMI) of between 28 and 30 or more. The BMI is a measure of the amount of body fat based on height and weight. A normal BMI is between 18.5 and 24.9. These numbers represent kilograms per square meter.


Xenical should be used with the utmost caution by people who have kidney disease, diabetes (insulin dose may need adjusting), epilepsy (absorption of antiepileptic drugs may be altered) and hypothyroidism--or underactive thyroid (hypothyroid medication, like levothyroxine, may become less efficacious). Also, contraceptive pills and fat-soluble vitamins, like A, D and E, might not get absorbed as effectively. Consult with your doctor about vitamin supplementation if you are taking Xenical.


People on the hypothyroid drug levothyroxine have an underfunctioning thyroid. The thyroid is a gland--shaped like a bow tie--that's located at the front of your throat. The hormones it emits control your body's use of energy. With hypothyroidism comes a low emission of thyroid hormones and decreased energy, in addition to susceptibility to high cholesterol and heart attack. Levothyroxine is sometimes prescribed as a replacement for the hormone your thyroid naturally produces, but people on levothyroxine who seek prescription weight-loss remedies must use caution.


Xenical has been shown to hamper the absorption of levothyroxine. Consult your doctor if you have hypothyroidism and are taking medication, particularly if you take levothyroxine. Also, should you notice your levothyroxine becoming less effective with the use of Xenical, tell your doctor right away. Most people take levothyroxine for life, which means they must be very cognizant of all the other medications they might ingest. The manufacturer of levothyroxine specifically states that people should not use the drug to treat weight problems.

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