This article studying rats has shown that the brain's receptors for thyroid hormones predominate in those that are more sensitive to T4 than others elsewhere in the body. This is just to let those who have no thyroid and also take T3 only that this might not be optimal. The article is downloadable:
Frontiers in Endocrinology (Lausanne). 2014; 5: 40.
Published online 2014 Mar 31. doi: 10.3389/fendo.2014.00040
Thyroid Hormones, T3 and T4, in the Brain
Amy C. Schroeder1 and Martin L. Privalsky1,*