MYH6 and hyperthyroidism: Hyperthyroidism, by opposing CLFS, reduces the content of β-slow MyHC and increases the content of 2A and 2X MyHCs to 34% of MyHC pool (Haddad et al. 1997), while denervation plus hyperthyroidism further increases these isoforms (Haddad et al. 1997).