Hypothyroid Myopathy Workup
- Author: Divakara Kedlaya, MBBS; Chief Editor: Stephen Kishner, MD, MHA more...
Nerve conduction studies may show decremental response to low-rate nerve stimulation and incremental response to high-rate nerve stimulation, suggesting some neuromuscular junction abnormality.
Needle electromyography is usually normal or may be mildly myopathic. Myotonia may be present in rare cases.
Increased thyroid stimulating hormone (TSH) is seen in primary hypothyroidism, and decreased TSH is seen in pituitary-hypothalamic disease with secondary hypothyroidism.
Low free T4 and T3 levels, as well as a low total T4 level and a free T4 index, are seen. The degree of myopathy is not necessarily correlated to the degree of hypothyroidism.
The creatine kinase level can be very high (10-100 times greater than the normal level) in some patients, but it has no correlation with weakness. . The level returns to normal with adequate treatment.
Histologic findings on muscle biopsy are nonspecific. Pale central regions on nicotinamide adenine dinucleotide with accumulation of periodic acid-Schiff – positive material are seen. Decreased numbers of beta-adrenergic receptors are observed and are accompanied by glycogenolysis. Some muscle fiber atrophy is noted, and increased numbers of internal nuclei, glycogen aggregates, and (occasionally) deposition of mucopolysaccharides in the connective tissue are characteristic of hypothyroid myopathy.
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