Hypothyroidism is a condition of insufficient thyroid hormone
production that causes slowed metabolism. Table
223-1 lists common causes of hypothyroidism. Hypothyroidism
is particularly common in areas of iodine deficiency, particularly
inland areas where there is no access to marine foods. In iodine-sufficient
areas, chronic autoimmune (Hashimoto) thyroiditis is the leading
cause of primary hypothyroidism. Hypothyroidism
has also been estimated to occur in 1% to 32% of
patients taking amiodarone.2 Individuals who have
thyroid peroxidase antibodies and those who have thyroid-stimulating
hormone (TSH) values that are in the upper normal range are at increased
risk for developing hypothyroidism.
223-1 Common Causes of Hypothyroidism |Favorite Table|Download (.pdf)
223-1 Common Causes of Hypothyroidism
|Primary Hypothyroidism (disorders at
thyroid gland)||Secondary Hypothyroidism (disorders at hypothalamic-pituitary
|Autoimmune etiologies (Hashimoto)||Panhypopituitarism|
|Thyroiditis (subacute, silent, postpartum)*||Pituitary adenoma|
|Iodine deficiency||Infiltrative causes (e.g., hemochromatosis,
|After ablation (surgical, radioiodine)|
|After external radiation||Tumors impinging on the hypothalamus|
|Infiltrative disease (lymphoma, sarcoid, amyloidosis,
tuberculosis)||History of brain irradiation|
|Infection (e.g., tuberculosis)|
|Drugs affecting thyroid gland function|
|Iodine (in patients with preexisting autoimmune
Thryroxine is the major form of thyroid hormone. The ratio of thyroxine to triiodothyronine released in the blood is about 10:1. Peripherally, thyroxine is converted to the active triiodothyronine, which is three to four times more potent than thyroxine. The half-life of thyroxine is 7 days, and the half-life of triiodothyronine is about 1 day.
Myxedema coma (also called myxedema crisis)
is the end spectrum of severe hypothyroidism and is an emergency.
It is a clinical syndrome of multi-organ and metabolic dysfunction
resulting from severe untreated hypothyroidism, and precipitated
by a number of stressors. The condition is a severe, life-threatening
decompensation of a hypothyroid patient heralded by mental status
changes, hypotension, and hypothermia. Almost 90% of cases
occurs in elderly women during the winter.
Subclinical hypothyroidism is
more prevalent and can be seen in as many as 15% of older
women. In the United States National Health and Nutrition Examination
Survey the prevalence of overt hypothyroidism was
found to be 0.3%, whereas the prevalence of subclinical hypothyroidism was
found to be 4.3%.3
Euthyroid individuals, who have detectable thyroid
autoantibodies, are at increased risk for developing overt hypothyroidism.
Up to 15% of elderly women have thyroid autoantibodies.4
Primary hypothyroidism is caused by the
intrinsic dysfunction of the thyroid gland, and this is the most
common type. Secondary hypothyroidism is caused
by a deficiency of TSH from the
pituitary gland or deficiency of thyrotropin-releasing hormone from
the hypothalamus. The distinction between primary and secondary
hypothyroidism is important because TSH administration is ineffective
in primary hypothyroidism.
The common features of hypothyroidism are listed in ...