The equivalency of two L-thyroxine preparations.

Area of Science:

• Endocrinology
• Pharmacokinetics
• Thyroid Hormone Replacement Therapy

Background:

• Previous studies indicated bioavailability differences between synthetic L-thyroxine formulations.
• Recent manufacturing changes rendered prior data obsolete, necessitating updated research.

Purpose of the Study:

• To compare the bioavailability of two principal brands of synthetic L-thyroxine: Synthroid and Levothroid.
• To assess potential differences in thyroid hormone levels and response to thyrotrophin-releasing hormone between the two brands.

Main Methods:

• A crossover study design was employed.
• Thirty-four patients with hypothyroidism were enrolled, with participants switching between Synthroid and Levothroid (or vice versa) every 6 weeks.
• Thyroid hormone levels and thyrotrophin levels post-thyrotrophin-releasing hormone administration were measured.

Main Results:

• While routine thyroid hormone measurements showed no significant differences, combined data revealed significantly higher free thyroxine levels in patients treated with Synthroid.
• Lower thyrotrophin values at 15 and 30 minutes after thyrotrophin-releasing hormone administration were observed with Synthroid treatment.
• Despite similar doses, the two brands demonstrated differences in bioavailability, indicating they are not bioequivalent.

Conclusions:

• Current formulations of Synthroid and Levothroid, although similar, are not bioequivalent.
• Clinical implications regarding brand switching and therapeutic equivalence warrant further consideration.