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[hypo- + thyroid + -ism]
The clinical consequences of inadequate levels of thyroid hormone in the body. Chronic or acute thyroid deficiency causes diminished basal metabolism, intolerance of cold temperatures, fatigue, mental apathy, physical sluggishness, constipation, muscle aches, dry skin and hair, and coarsening of features. Collectively, these symptoms are called myxedema. In infancy, inadequate levels of thyroid hormone cause cretinism.
SEE: thyroid function test

A little less than 5% of the population has hypothyroidism. In the U.S., the prevalence is lower in African-Americans than in other groups.

Most patients with hypothyroidism either have Hashimoto (autoimmune) thyroiditis or have undergone treatment for hyperthyroidism with thyroidectomy or radioactive iodine. Occasionally, hypothyroidism is drug induced, e.g., in patients treated with antithyroid drugs (propylthiouracil) or the antiarrhythmic agent amiodarone. In areas where salt is not iodized, hypothyroidism may result from dietary iodine deficiency. Hypothyroidism may occasionally result from inadequate stimulation of the thyroid gland by the anterior pituitary gland or from inadequate release of thyrotropin-releasing hormone by the hypothalamus.

Long before the symptoms of hypothyroidism become obvious, the condition can be diagnosed with thyroid function tests. The plasma thyroid-stimulating hormone (TSH) test is used to screen for the disease; if it is high, hypothyroidism is likely to be present. Other tests, including a low serum free thyroxine (T4 index), confirm the diagnosis.

For most patients, the lifelong administration of thyroid hormone at a dose of approx. 1.6 µg/kg/day of oral levothyroxine restores normal metabolism and well-being. Failure to treat hypothyroidism inevitably results in myxedema, eventual coma, or death. Drug-induced hypothyroidism sometimes requires no treatment other than discontinuation of the offending agent or adjustment of its dose.

The patient is assessed for indications of decreased metabolic rate; easy fatigability; cool, dry, scaly skin; hypercarotenemia; loss of hair and eyebrows; brittle nails, puffiness in the face and periorbital edema; paresthesias; ataxias; intolerance of cold; bradycardia; reduced cardiac output; slow pulse rate, poor peripheral circulation, aching muscles, and joint stiffness; changes in bowel habits; irregular menses; and decreased libido. Reflexes, esp. in the Achilles tendon, show delayed relaxation time. In acute hypothyroid crisis (myxedema coma), vital signs, fluid intake, urine output, weight, and neurological status are monitored.

Chronic management includes the prescription of long-term hormone replacement. The patient's activity level is increased gradually as treatment proceeds; adequate rest is a continual priority to limit fatigue and to decrease myocardial oxygen demand. The patient should wear or carry a medical identification device describing the condition and its treatment and carry medications at all times. Desired outcomes include understanding of and cooperation with the treatment regimen, restoration of normal activity level, absence of complications, and restoration of psychological well-being.

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