With an estimated 120 million prescriptions filled each year, the thyroid medicine levothyroxine (marketed as Synthroid ®) is one of the most popular prescription medicines in the United States. Most patients who suffer from hypothyroidism — a shortage of thyroid hormone, usually caused by a damaged or missing thyroid gland — respond favorably to treatment with this drug.

Nearly 15 percent of patients, however, get only limited benefit from levothyroxine. Their symptoms, such as fatigue, weakness, weight gain, cramps, irritability and often memory loss, persist, even among patients who take this affordable medicine consistently.

On Oct. 23, 2018 the Journal of Clinical Investigation posted an ‘in-press preview” of this multi-institutional study, describing how one dysfunctional protein can disrupt the efficacy of this otherwise highly effective treatment. The damage is caused by an inherited mutation in a critical enzyme that puts nearly one out of five patients at risk for not being able to experience the established benefits of levothyroxine.

“Even though they take their medications, many hypothyroid patients continue to have problems,” said thyroid specialist Antonio Bianco, MD, PhD, professor of medicine at the University of Chicago and senior author of the study. “They lack energy, they feel unfocused and they have trouble losing weight. They are properly taking thyroid hormone but their problems aren’t going away. They get frustrated when they see little change, only limited improvement. Many change physicians multiple times, sometimes more than 10 times.”

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