Thyroxine is a common medication with over 1,600,000 prescriptions supplied in Australia during the 2013/2014 financial year. Thyroxine is prone to instability, which is concerning due to its reasonably narrow therapeutic index. In 2004, the Therapeutic Goods Administration (TGA) responded to reports of inconsistent stability by mandating refrigerated storage for all thyroxine tablets in Australia. Tablets were then required to be stored between 2°C and 8°C, although a single blister strip may be stored below 25°C for up to 14 days.

These revised storage conditions may be inconvenient for some patients and have a negative effect on their compliance. Eltroxin® is a new formulation of thyroxine that is stable at room temperature. Eltroxin® tablets, manufactured by direct compression instead of wet granulation, retain thyroxine in the pentahydrate state due to the absence of a drying step in their production. This increases tablet stability so that potency is maintained throughout the 24-month shelf-life when stored below 25°C and protected from light.

Due to the differences in formulation and manufacturing processes, Eltroxin® is not bioequivalent to other thyroxine brands. Studies demonstrate an 11% lower bioavailability compared to Oroxine® and Eutroxsig®. Therefore, patients should not change formulations unless they can be closely monitored by their physician as dose adjustments may be required.

References:

  1. Eltroxin® (thyroxine) Australian approved product information. St Leonards: Aspen Pharma Pty Ltd. Approved December 2014.
  2. Therapeutic Goods Administration. Australian public assessment report for thyroxine sodium. Commonwealth of Australia. Woden, 2014.

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