The Equivalency of Two L-Thyroxine Preparations

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Abstract
Differences in blood levels of thyroid hormones have been reported when patients taking synthetic L-thyroxine were shifted from 1 manufactured form of the hormone to another. Because recent formulation changes have rendered these earlier data obsolete, a crossover study was done to determine differences in bioavailability of the 2 principal brands of thyroxine, Synthroid and Levothroid. Patients (20) received Synthroid and then Levothroid, and 14 received Levothroid and then Synthroid. Patients received each drug for 6 wk. Although no significant differences were seen in routine thyroid hormone measurements, combined data from both groups showed a significantly higher free thyroxine level in the patients treated with Synthroid, as well as lower thyrotrophin values at 15 and 30 min after administration of thyrotrophin-releasing hormone. Equal doses of the currently manufactured forms of these 2 products are similar but not bioequivalent.
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