Deficiency of thyroid hormone leads to a state called hypothyroidism. Common causes of hypothyroidism include the following:
Treatment with radioactive iodine (I 131) for Graves disease
Hashimoto disease - An autoimmune process in which lymphocytic infiltration and fibrous tissue accumulation cause replacement of normal thyroid tissue
Drug-induced hypothyroidism - Known to occur with amiodarone and iodine (ie, Wolff-Chaikoff effect)
Hereditary disorders of the iodothyronine synthesis pathway (thyroxine [T4] and triiodothyronine [T3])
Pituitary tumors and related surgical resections
Hypothyroidism can cause several symptoms, ranging from mild (eg, fatigue, weight gain, cold intolerance, mental slowing, muscle cramping) to severe (eg, heart enlargement, myxedema coma [rare]).
Without regard to the cause of hypothyroidism, neuromuscular and musculoskeletal manifestations can be observed in many patients with the condition.  These manifestations can occur at any time in the hypothyroidism disease process. Usually mild, they include weakness, pain, aching, and stiffness.
Hypothyroid myopathy typically manifests as polymyositis-like myopathy with proximal muscle weakness and an increased creatine kinase level. [2, 3] However, it sometimes manifests as muscle enlargement (pseudohypertrophy); in adults, this condition is called Hoffman syndrome. [4, 5, 6] In children with hypothyroid disease (cretinism), a pattern of proximal weakness and diffuse muscle enlargement is known as Kocher-Debr é -S é m é laigne syndrome.
Several case reports describe rhabdomyolysis associated with hypothyroidism.  In these cases, the hypothyroidism is thought to have predisposed individuals to rhabdomyolysis.
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Resource Center Hypothyroidism
Resource Center Thyroid Disease
Hypothyroidism causes a constellation of changes in the body. The lack of thyroid hormone results in slowed or reduced metabolic function, such as decreased protein turnover and impaired carbohydrate metabolism. These metabolic changes occur in many organ systems, including the muscles. Pain with muscle exertion is characteristic of defective carbohydrate metabolism.
Slowed muscle contraction and relaxation, known as hypothyroid myopathy, may be caused by a shift in the distribution of muscle fiber types from fast-twitch fibers to slow-twitch fibers. A reduction in muscle mitochondrial oxidative capacity and beta-adrenergic receptors, as well as the induction of an insulin-resistant state, may result in these changes.
Evidence from a study by Sinclair and colleagues suggests that a decrease in muscle carnitine in patients with either hypothyroidism or hyperthyroidism may contribute to thyroid myopathy. 
The global inhibition of the main oxidative pathways (substrate incorporation, substrate oxidation) and of the respiratory chain within cells also may cause myopathic symptoms. A diminished energetic consumption is related partially to a transition in the myosin isoforms, which express a slower adenosine triphosphatase, and to an impairment of the transsarcolemmal transports. All of these factors may contribute to muscle weakness, fatigue, and exertional pain.
Exercise intolerance could be due to an abnormal recruitment of several metabolic pathways, such as glycolysis, related to the mitochondrial metabolism impairment. An abnormal accumulation of protons and monovalent phosphate ions (which are involved in the actin-myosin interaction), as well as abnormal Ca++ metabolism, also may cause reduced exercise tolerance. [9, 10]
Hyponatremia often is seen in patients with hypothyroidism. The decreased number of Na+/K+ adenosine triphosphatase–dependent pumps could imply an abnormal intracellular Na+ level and explain frequent disorders of membrane excitability.
Myoedema is a phenomenon in which mounding up of muscle tissue occurs after light percussion. Myoedema occurs in approximately one third of patients with hypothyroidism, but it is not entirely specific for hypothyroidism. In one study, myoedema was elicited in 40% of overt hypothyroid patients, while historical clinical features of myopathy were present in 73%.  Myoedema is thought to be caused by delayed Ca++ reuptake by the sarcoplasmic reticulum, which also prolongs muscle contraction. Although not proven, this type of prolongation of muscle contraction is also thought to cause muscle hypertrophy.
In North America, acquired impairment of thyroid function affects about 2% of adult women and about 0.1-0.2% of adult men.
Neonatal hypothyroidism occurs with a frequency of 0.02% in the white population. In the black population, the frequency falls to 0.003%.
Of individuals with hypothyroidism, 30-80% manifest neuromuscular symptoms, depending on the severity of hypothyroidism. Weakness is observed in one third of patients with hypothyroidism. Carpal tunnel syndrome, although not part of the myopathy, is a peripheral nerve dysfunction found in 15-30% of patients with hypothyroidism. 
In a prospective cohort study done in The Netherlands, in newly diagnosed patients with hypothyroidism, 79% had neuromuscular complaints, 38% had clinical weakness (manual muscle strength testing) in one or more muscle groups, 42% had signs of sensorimotor axonal neuropathy, and 29% had carpal tunnel syndrome. 
Neonatal screening programs for congenital hypothyroidism in many areas of the world show that hypothyroidism is present in 1 out of every 4000 newborns. In iodine-deficient areas of the world, the incidence of hypothyroidism is 10- to 20-fold higher.
Mortality has not been shown to be increased in patients with hypothyroid myopathy. Morbidity is significantly increased, reflected in the performance of activities of daily living (ADL) and in patients' quality of life.
No race predilection has been established.
The incidence of hypothyroidism is much greater in women than in men. Myxedema coma is extremely rare, but its incidence is higher in elderly women, especially those older than 60 years.
Hypothyroidism is seen more commonly in women aged 40-60 years; however, it can be seen in persons of all ages.
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