MYH6 and hyperthyroidism: In the adult rat soleus muscle, hyperthyroidism suppresses β-slow MyHC and induces the expression of 2A and 2X MyHCs but not 2B MyHC (Li and Larsson 1997) while in a fast muscle, hyperthyroidism shifts MyHC expression in the direction 2A→2X→2B (Kirschbaum et al. 1990).