Muscular Disorders in Hyperthyroidism

In elderly patients, hyperthyroidism can cause a subacute proximal myopathy, without the prominent tachycardia or other obvious systemic signs that occur in younger patients. It can also cause myokymia (continuous quivering or undulating movement of muscle surface and overlying skin), acute bulbar myopathy, ocular myopathy, and, rarely, hypokalemic periodic paralysis.

Often, the initial symptoms are weakness of proximal limb muscles and increased muscle fatigue. Usually, the shoulder girdle and upper arm muscles are weaker than the leg muscles. In the legs, the iliopsoas muscle may be primarily affected. Muscle atrophy develops early and occasionally is pronounced. More than 15% of patients have prominent muscle twitches, often with increased tendon reflexes, which may mimic those of amyotrophic lateral sclerosis.

The diagnosis is suspected clinically and confirmed by thyroid function tests. Serum muscle enzyme levels are normal. Electromyography often demonstrates fasciculations and myopathic features in weak muscles. Muscle biopsy results are usually normal.

Prognosis and Treatment

The long-term prognosis is good if the hyperthyroidism is treated and the euthyroid state restored. During the first few weeks of treatment, propranolol 120 to 320 mg/day often reduces proximal muscle weakness. After the euthyroid state has been maintained for several months, muscle bulk and strength typically return to normal.