I'm sure they would compete for receptors, but you would not be saturating the receptors. I'm just going to use some simple numbers for an example. Say you have 10 RXR receptors. Normally 4/10 are used by the thyroid hormone at any given time. You add sesamin which uses another 4 receptors, so you are now using 8/10 receptors. Now if you add exogenous thyroid hormones into your system which would need another 4 receptors, you would need 12 receptors when you only have 10.
This is where either using Sesamin or T3 comes into my. Using just one won't compromise receptor competition, but using 2 compounds that compete for the same receptor at the same time would. Again, it all depends on dosages as well.