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Quantifying Amount of Adsorption of Levothyroxine by Percutaneous Endoscopic Gastrostomy Tubes

From the ¹ Division of Endocrinology, Diabetes and Metabolism, Department of Medicine and ² Department of Nuclear Medicine, Saint Vincents Catholic Medical Centers–Manhattan, New York Medical College, New York.

Address correspondence to: Iven Young, MD, Division of Endocrinology, Diabetes, and Metabolism, Saint Vincents Catholic Medical Centers–Manhattan, 130 West 12th Street, New York, NY 10011; e-mail: iyoung{at}nyc.rr.com.

Background: It has been the authors' clinical experience that hypothyroid patients who achieve a euthyroid state on a steady dose of oral levothyroxine often become hypothyroid over time if the medication is given via a feeding tube. The authors hypothesize that the tubing and enteral feeds may adsorb a significant percentage of the levothyroxine and thereby reduce its bioavailability. To the authors' knowledge, no previous research has been reported on this subject. They therefore performed an in vitro assessment of the degree of levothyroxine adsorption to quantify the amount of drug adsorbed to the percutaneous endoscopic gastrostomy (PEG) tube and how enteral tube feeds mitigate or exacerbate this adsorption. Methods: Using levothyroxine radiolabeled with an I¹²⁵ tracer, a known dose of levothyroxine was passed through 60 new PEG tubes. One-half of the tubes were pretreated with Jevity feeds, and the other half were not. The authors measured the activity of the radiolabeled levothyroxine before and after it had passed through the tubes and, using a subtraction analysis, inferred the amount of thyroxine left within the tube. Results: Tubes presoaked with feeds had a greater uptake in radioactivity by 326.4 cpm (95% confidence interval, 226.7-426.1), corresponding to a 45.08% relative increase in uptake compared with virgin PEG tubes without feeds. Conclusions: Although the authors found statistically significant differences in mean drug concentrations, they conclude that the amount of uptake of levothyroxine by PEG tubes and adsorption of levothyroxine by PEG tubes is probably clinically insignificant. The differences found may be attributed to the amount of drug lost during crushing and transfer.

Key Words: adsorption • levothyroxine • PEG tubes • hypothyroidism

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