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Review

Adult Hypothyroidism

In: Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000.
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Review

Adult Hypothyroidism

Wilmar M Wiersinga.
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Excerpt

Hypothyroidism is traditionally defined as deficient thyroidal production of thyroid hormone. The term primary hypothyroidism indicates decreased thyroidal secretion of thyroid hormone by factors affecting the thyroid gland itself; the fall in serum concentrations of thyroid hormone causes an increased secretion of TSH resulting in elevated serum TSH concentrations. Decreased thyroidal secretion of thyroid hormone can also be caused by insufficient stimulation of the thyroid gland by TSH, due to factors directly interfering with pituitary TSH release (secondary hypothyroidism) or indirectly by diminishing hypothalamic TRH release (tertiary hypothyroidism); in clinical practice it is not always possible to discriminate between secondary and tertiary hypothyroidism, which are consequently often referred to as central hypothyroidism. In rare cases, symptoms and signs of thyroid hormone deficiency are caused by the inability of tissues to respond to thyroid hormone by mutations in the nuclear thyroid hormone receptor TRß; this condition, known as thyroid hormone resistance (see Ch. 16 ), is associated with an increased thyroidal secretion of thyroid hormones and increased thyroid hormone concentrations in serum in an attempt of the body to overcome the resistance to thyroid hormone. Mutations in other genes involved with extrathyroidal metabolism and action of thyroid hormones in target tissues may also cause a hypothyroid state. Such cases could be labelled as peripheral (extrathyroidal) hypothyroidism. It thus seems more appropriate to define hypothyroidism as thyroid hormone deficiency in target tissues, irrespective of its cause.

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