There are three TR-α receptor splice variants encoded by the THRA (thyroid hormone receptor alpha) gene and three TR-β isoform splice variants encoded by the THRB (thyroid hormone receptor beta) gene. Of these variants, thyroxine is only able to bind to four of them: TR-α1, TR-β1, TR-β2, and TR-β3.
In the context of thyroid hormone receptors, there are three main types of TR-α receptors (TR-α1, TR-α2, TR-α3) encoded by the THRA gene, as well as three forms of TR-β receptors (TR-β1, TR-β2, TR-β3) encoded by the THRB gene. Notably, thyroxine can only bind to four specific variants: TR-α1, TR-β1, TR-β2, and TR-β3.