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.
The THRA gene produces three different types of TR-α receptors, while the THRB gene generates three distinct forms of TR-β receptors. Notably, thyroxine can only bind to four specific variants: TR-α1, TR-β1, TR-β2, and TR-β3.