by N.Morgan
The effects of medications, supplements, excessive iodine intake and other exogenous substances including endocrine-disrupting chemicals can influence multiple thyroid effects that clinicians should recognize, including thyroid dysfunction and altered thyroxine absorption, according to a speaker at the AACE Annual Scientific and Clinical Congress.
Many commonly used drugs and popular over-the-counter supplements, as well as newer cancer treatments such as immune checkpoint inhibitors, can interfere with thyroid function test measurement and thyroid hormone metabolism, disrupt the thyroid axis and even trigger autoimmune thyroid disease, Gregory A. Brent, MD, professor of medicine and physiology in the division of endocrinology at the David Geffen School of Medicine at UCLA, said. Popular OTC dietary supplements such as biotin, a water-soluble vitamin often marketed for hair and nail growth, can interfere with a range of thyroid assays that rely on the interaction of biotin and streptavidin, Brent said.
“Biotin, a nutritional supplement used by many, can interfere with thyroid function tests and other hormone assays,” Brent told Endocrine Today. “Patients who have ingested large amounts of biotin may have abnormal test results that look like thyroid disease, but the disease is not present.”
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In these patients, cessation of biotin for 1 to 3 days can rapidly reverse the interference and thyroid function tests should return to normal, Brent said.
Other commonly prescribed therapies, such as glucocorticoids, dopamine or the vitamin A derivative bexarotene, can inhibit or block thyroid-stimulating hormone release or suppress pituitary TSH-beta promoter, Brent said.
Thyroxine absorption
Several agents and conditions have an impact on levothyroxine therapy and the interpretation of thyroid tests, Brent said, including common drugs like proton pump inhibitors, ferrous sulfate and calcium salts, which can interfere with drug absorption. The food or drink a patient consumes with the levothyroxine therapy — for example, milk and coffee — can also impact absorption, he said.
“Thyroxine is best absorbed in an acidic environment, so anything you do that raises the gastric pH will affect the dissolution of the levothyroxine tablet,” Brent said.
Additionally, Brent said, many patients have altered thyroxine absorption, but may also have poor or erratic adherence. A levothyroxine absorption test can identify patients with true absorption problems, he said.
Cancer treatments, thyroid dysfunction
Cancer treatment, particularly the newer immune checkpoint inhibitors, is associated with a wide range of endocrine disorders, and the thyroid is the most commonly affected, Brent said.
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