Muscle involvement in a variety of forms is a frequent problem in both congenital and adult-onset hypothyroidism. This topic will discuss the clinical manifestations, pathogenesis, and treatment of hypothyroid myopathy. Other neurologic manifestations of hypothyroidism are discussed separately. (See "Neurologic manifestations of hypothyroidism" and "Clinical manifestations of hypothyroidism".)
The Kocher-Debre-Semelaigne syndrome describes infants with typical features of cretinism associated with diffuse muscular hypertrophy and muscle weakness that is predominantly proximal. These infants have motor and cognitive developmental delay, constipation, myxedema, enlarged tongue, and coarse hair and skin typical of cretinism. Despite a very muscular, almost muscle-bound appearance, they are in fact weak and often have difficulty with sitting and head control . (See "Iodine deficiency disorders", section on 'Cretinism'.)
Serum creatine kinase (CK) and other muscle enzymes are often elevated in these infants. The electromyogram may be normal or show scattered low amplitude motor unit potentials in involved muscles. Muscle biopsy shows variable muscle fiber size with predominantly type I fiber atrophy. There is also increased interstitial connective tissue related to the duration of the hypothyroidism and abnormal glycogen accumulation. Muscle fiber necrosis and inflammation are not prominent features of this disorder .
Treatment with thyroid hormone results in prompt improvement in muscle strength and normalization of serum CK concentrations, even in infants with longstanding congenital hypothyroidism .
Muscle involvement in adults with hypothyroidism is common; in one series, 79 percent of patients with hypothyroidism had muscle complaints (weakness, cramps, myalgias) . Patients with more severe or longstanding untreated hypothyroidism are more likely to develop clinically significant muscle disease.