"Some studies have related hypothyroidism to muscle dysfunction... Furthermore, studies ... demonstrated that thyroid hormone action on skeletal muscles affects mainly type-I muscle fibers, which promote slow contractions and are most prevalent in the postural muscles recruited during prolonged effort" 
Also, not sure where you are in the world but your doctor telling you your thyroid hormone levels are ok unfortunately may not necessarily mean they are ok... There is a controversy about reference ranges amongst experts , and I think it's wise to pay attention to that.
BTW, I think this is also worth highlighting (from  above):
"Different metabolic changes, yet with similar consequences, may therefore be observed in both hypothyroid and hyperthyroid patients. In the first case, fatigue is directly related to deficient action of thyroid hormones. In the latter,
however, the cause is mainly depletion of muscle energy substrate due to high metabolic demand."