A small study in 1999 assessed the benefits of T4/T3 combination but despite being a good statistical analysis the results were disappointing. The authors found no difference in symptoms such as pain, depression, anxiety or fatigue. The subsequent recommendation was to continue just on T4 replacement.

More recently a Danish study did find a significant difference when patients were given the T4/T3 combination compared to T4 alone. Almost half their patients preferred the combination, some preferred only T4 and some found no difference. This study raised the question as to whether the previous negative studies were correct and whether they targeted the wrong patient group.

The European Thyroid Association set up a study protocol to examine these points further. A task force was set up to study results of trials and to assess safety based on research around Europe. Results are published in the European Thyroid Journal.

There were some positive results in some patients using T4/T3 in combination and currently there is further work to examine exactly who would benefit. Also there are research trials which are looking at the best dose combinations and ratios within the T4/T3 replacement therapy.