Why does the new Euthyrox require dose adjustment when only the composition of the excipients has changed?

I am not aware of information about the change in the composition of the drug Euthyrox N. The last change took place in 2019. Currently, the change in the composition of excipients is undergoing its replacement, i.e. Letrox. The manufacturer informs that there has been a change in the composition of the excipients. This may result in a change in the absorption of the drug in some patients, in addition, thanks to the modification of the composition, a better durability of the active substance within the drug has been obtained. Due to the fact that Letrox contains very small doses of levothyroxine - measured in micrograms - even a small difference in absorption and shelf life can have a significant impact on the final therapeutic effect. When placing the drug on the market, the manufacturer performs appropriate safety and equivalence studies of the old and current versions of the drug, however, in order to ensure the highest quality of therapy, it recommends additional caution and sensitivity to possible new symptoms after changing the version of Letrox. If they occur, the dose of Letrox may need to be adjusted.