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Literature on Hypothyroidism

Hypothyroidism and Myxedema Coma

Erik D Schraga,MD

eMedicine Endocrine & Metabolic http://www.medscape.com Updated: Apr 19, 2010



Background

Hypothyroidism is a clinical syndrome in which the deficiency or absence of thyroid hormone slows bodily metabolic processes. Symptoms can manifest in all organ systems and range in severity based on the degree of hormone deficiency. The disease typically progresses over months to years but can occur quickly following cessation of thyroid replacement medication or surgical removal of the thyroid gland.
The term myxedema refers to the thickened, nonpitting edematous changes to the soft tissues of patients in a markedly hypothyroid state. Myxedema coma, a rare, life-threatening condition, occurs late in the progression of hypothyroidism. The condition is seen typically in elderly women and is often precipitated by infection, medication, environmental exposure, or other metabolic-related stresses. Because rapid confirmatory laboratory tests are oftenunavailable, the diagnosis may be made on clinical grounds with treatment started promptly.
Treatment of myxedema coma requires potentially toxic doses of thyroid hormone, and mortality rates exceeding 20% have been reported even with optimum therapy.
For more information, see Medscape's Hypothyroidism Resource Center.

Pathophysiology

Thyroid hormone is secreted in response to stimulation of the thyroid gland by thyroid-stimulating hormone (TSH) from the anterior pituitary gland. TSH is released through the action of thyrotropin-releasing hormone (TRH) from the hypothalamus.
Hypothyroidism can be caused by permanent loss or atrophy of functional thyroid tissue (primary hypothyroidism), insufficient stimulation of a normal thyroid gland by as a result of hypothalamic or pituitary disease (secondary hypothyroidism, often accompanied by compensatory thyroid gland enlargement), or a defect in the TSH molecule (control hypothyroidism).
Primary hypothyroidism accounts for approximately 90-95% of hypothyroidism, with a predominantly autoimmune-mediated etiology. TSH hypersecretion produces excessive thyroid tissue, resulting in goiter formation. Surgical and radiation ablation account for a large percentage of acquired cases of hypothyroidism. Congenital abnormalities, malignancies, and infiltrative disorders including amyloidosis and sarcoidosis can also lead to the disease. Iodine deficiency is rarely responsible for hypothyroidism in developed countries; however, it remains the primary cause worldwide.
Suprathyroidal disorders including hypopituitarism and hypothalamic lesions account for fewer than 10% of cases. Rarely, peripheral resistance to thyroid hormone may occur.
The congenital absence or deficiency of thyroid tissue may result in cretinism, a neurodevelopmental disorder characterized by lethargy, poor peripheral circulation, constipation, and goiter. Because infants are asymptomatic, neonatal screening is vital to prevent permanent sequelae.