Vitamin C improves oral absorption of the thyroid hormone replacement drug levothyroxine, according to a new study. The results will be presented Monday, June 16, at The Endocrine Society’s 90th Annual Meeting in San Francisco.

Some patients taking this oral drug to treat an underactive thyroid have difficulties with its absorption into the bloodstream, said the coauthor who presented the findings, Sandra Licht, MD, an endocrinologist at the University of Buenos Aires in Argentina. Paula Antunez, also of the University of Buenos Aires, is the study’s principal author.

With inadequate drug absorption, the patient’s abnormally high blood levels of thyroid-stimulating hormone (TSH) fail to decrease, and symptoms continue, such as fatigue, depression and weight gain. When this occurs, the patient usually needs a higher dose and further testing, she said.

Certain medications and digestive diseases interfere with the absorption of levothyroxine, but often malabsorption occurs for unknown reasons, Licht said.

Past research has shown that taking an acidic substance (hydrochloric acid) at the same time as the levothyroxine tablet may enhance the drug’s absorption. Therefore, the authors studied whether simultaneous use of vitamin C (ascorbic acid) would improve dissolution of the levothyroxine tablet and thus absorption.

They studied 11 patients (eight women and three men) who were taking higher-than-average doses of levothyroxine but not reaching their target TSH levels, as found by two different blood tests. None of the patients had factors that interfered with levothyroxine absorption, such as celiac disease or use of calcium or iron supplements or antacids. For 6 weeks, patients took 1 gram of vitamin C with their levothyroxine pill. They diluted a vitamin C tablet in about 7 ounces of water.

After six weeks, all 11 patients had decreased TSH levels, with an average reduction of about 69 percent. Nine of the patients achieved the desired level of TSH.

The decrease in TSH indirectly shows that levothyroxine absorption increased, Licht explained. She said that studies in larger numbers of patients are needed to confirm their finding.

Although the patients in their study diluted the vitamin C pill in water, Licht said she expects that swallowing the pill directly would have the same effects on drug absorption.

“Patients with difficulties in the absorption of levothyroxine should consider talking to their doctors about co-administration of vitamin C with levothyroxine,” Licht said, “especially if they have a history of kidney stones, anemia or acid reflux.”

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Founded in 1916, The Endocrine Society is the world's oldest, largest, and most active organization devoted to research on hormones and the clinical practice of endocrinology. Today, The Endocrine Society's membership consists of over 14,000 scientists, physicians, educators, nurses and students in more than 80 countries. Together, these members represent all basic, applied, and clinical interests in endocrinology. The Endocrine Society is based in Chevy Chase, Md. To learn more about the Society, and the field of endocrinology, visit our web site at www.endo-society.org.