VANCOUVER, B.C. — Bioavailability of even the newer rapid-release formulation of levothyroxine known as Levoxyl is reduced 40% by taking the tablets close to a meal, Michael J. Lamson, Ph.D., said at the annual meeting of the American Thyroid Association.
The hope was that Levoxyl's rapid release might permit a new flexibility in dosing, allowing patients to take the drug in proximity to meals instead of on an empty stomach, as recommended for L-T4 therapy. But that wasn't the case in the first large study to assess the effects of a meal on oral absorption of L-T4 using the Food and Drug Administration's formal bioequivalence methodology, said Dr. Lamson of King Pharmaceuticals R&D in Cary, N.C.
He reported on 48 healthy participants in a randomized, three-way crossover trial. They took two 300-mcg tablets of Levoxyl on three occasions, each separated by a 35-day washout. One dose was taken under fasting conditions. The second dose was taken 10 minutes before eating a meal and the third dose was taken just after a meal.
Bioavailability of L-T4 was reduced by 40% regardless of whether subjects took the drug shortly before or immediately after the meal. This is a clinically significant finding, because it is well established that even small changes in L-T4 bioavailability can have a profound impact upon the success of oral replacement therapy.
In clinical terms, this means that taking rapid-release L-T4 in proximity to a meal or a medication known to interfere with L-T4 renders a 100-mcg dose of Levoxyl equivalent to a 60-mcg dose, Dr. Lamson said. Patients who have been taking their L-T4 with meals may require a dose correction of up to 40%.