Hypothyroidism can affect the central and peripheral nervous systems at multiple levels, producing a diverse array of neurologic symptoms and signs. Clinicians should be particularly aware of the diagnostic and management issues related to myxedema coma, dementia, myopathy, and polyneuropathy. Carpal tunnel syndrome may be the most common neurologic abnormality associated with hypothyroidism. The importance of hypothyroidism as a “reversible cause of dementia” remains unclear. The diverse manifestations of Hashimoto encephalopathy, a syndrome that appears to be inflammatory rather than a direct result of inadequate thyroid hormone levels, are important to recognize because the syndrome typically responds to high-dose steroid treatment.
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• Hypothyroidism can affect practically every level of the central and peripheral nervous system.
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• Although coma due to severe hypothyroidism (myxedema coma) is rare, it should always be considered in comatose patients without a clear cause because it requires rapid and specific treatment.
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• Carpal tunnel syndrome is the most common peripheral nerve manifestation of hypothyroidism.
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• An elevated thyrotropin level is the key diagnostic finding in primary hypothyroidism, which is the most common form of hypothyroidism.
Historical note and terminology
Hypothyroidism is the clinical condition that results from inadequate synthesis of thyroid hormone. Myxedema is a state of severe hypothyroidism. William Gull published a detailed description of a patient with myxedema in 1874 and noted that the disorder resembled cretinism. In 1878 William Ord described 5 additional patients and proposed the designation "myxedema" because he was impressed with the nonpitting edema and gelatinous skin observed in these patients. In 1883 the Clinical Society of London named a committee to study myxedema, and in 1888 this committee published an analysis of 109 cases, noting a characteristic destruction of the thyroid gland in autopsied cases. The report also included the observation that a similar state could be produced in monkeys by removing the thyroid gland. An extract of sheep thyroid was first administered to a myxedematous patient by subcutaneous injection in 1891, and the first oral administration was attempted in 1892. Ground sheep thyroid is said to have been used as a common remedy for a condition resembling myxedema in fourth century China (160). The proposal that the Mona Lisa depicts a woman with postpartum hypothyroidism is probably impossible to prove (127).
Even the earliest descriptions of myxedema documented the frequent and prominent neurologic manifestations. With the identification of thyroxine in 1915 and triiodothyronine in 1952, it became possible to identify the neurologic consequences of less florid states of hypothyroidism.
There has been some inconsistency in the use of the term “myxedema.” Some use it as a synonym for hypothyroidism in general, others as a designation for severe hypothyroidism, and still others as a label for the doughy induration of the skin that occurs in severe hypothyroidism. Cretinism denotes a condition of abnormal development that results from congenital hypothyroidism.