**4. Conclusions**

The tablet analysis shows that the thyroid hormone supplements contain T3 and T4 content corresponding to the stated dosage. The dissolution testing profiles and kinetic of dissolution plots show that there is consistent solubilization of the active pharmaceutical ingredient across both singleand dual-component batches for thyroid hormone supplements. Therefore, it is concluded that the use of simulated in vitro gastric intestinal fluids has no influence on dual-component thyroid hormone supplement extraction and recovery. It was also noted, from the dissolution testing profile, that there was minimal solubilization of T3 and T4 in the gastric fluid. However, rapid release of the active compounds was observed within 15 min of the addition of the intestinal fluid.

**Author Contributions:** Conceptualization, J.R.D., M.G.C. and S.L.B.; methodology, S.L.B.; validation, S.L.B.; formal analysis, S.L.B.; investigation, S.L.B.; resources, S.L.B., J.R.D. and M.G.C.; data curation, S.L.B.; writing—original draft preparation, S.L.B.; writing—review and editing, J.R.D., S.L.B. and M.G.C.; supervision, J.R.D. and M.G.C.; project administration, J.R.D.

**Funding:** This research received no external funding.

**Acknowledgments:** We acknowledge GlaxoSmithKline for the donation of the dissolution bath to Northumbria University. We also gratefully acknowledge financial support from Northumbria University.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
