Hypothyroidism is a common medical disorder that is easily managed by replacement treatment with synthetic thyroxine. About 70 000 New Zealanders have hypothyroidism and take thyroxine replacement treatment. Since 1973 the only thyroid hormone replacement drug approved and funded by the government for use in New Zealand was the Eltroxin brand, made by GlaxoSmithKline. In 2007 the company moved the manufacture of Eltroxin from Canada to Germany. This resulted in a change in the tablets’ inert ingredients: the new formulation differed in markings, size, and colour and—according to some reports—also in taste and rate of dissolution on the tongue. The active ingredient (thyroxine) remained unchanged and continued to be made in Austria.

In 2007 and 2008 New Zealand pharmacies changed to the new formulation of Eltroxin. The old formulation had been used for more than 30 years without problems; but after the new tablets were introduced the rate of adverse event reporting rose nearly 2000-fold—from 14 reports in 30 years to more than 1400 in 18 months. What had happened? And does this incident provide important lessons for future formulation changes and migration to generic drugs?