Oral thyroxine: variation in biologic action and tablet content.

Area of Science:

• Endocrinology
• Pharmacology

Background:

• Primary hypothyroidism requires consistent oral thyroxine replacement therapy.
• Variability in oral thyroxine formulations can impact patient treatment outcomes.

Purpose of the Study:

• To compare the bioavailability and biologic activity of two oral thyroxine preparations, Levothroid and Synthroid.
• To investigate the impact of thyroxine content variations on serum thyroxine and thyrotropin levels.

Main Methods:

• Thirty-two patients with primary hypothyroidism were administered Levothroid and Synthroid.
• Serum thyroxine levels were measured as an index of bioavailability.
• Serum thyrotropin levels were measured as an index of biologic activity.
• Direct measurement of thyroxine content in both tablet formulations was performed.

Main Results:

• All patients exhibited lower serum thyroxine levels with Synthroid compared to Levothroid.
• Fifteen patients showed elevated serum thyrotropin levels when taking Synthroid, indicating reduced biologic activity.
• Direct tablet analysis revealed Synthroid contained 20% to 30% less thyroxine than stated.

Conclusions:

• Synthroid's decreased efficacy in hypothyroidism is attributed to lower-than-stated thyroxine content.
• Ensuring oral thyroxine potency and consistent biologic action is crucial.
• Cooperation between manufacturers, regulatory bodies, and clinicians is recommended to standardize oral thyroxine products.