Hypothyroidism is a clinical syndrome caused by insufficient thyroid hormone production, which slows cell metabolism. Hypothyroidism is common in areas where iodine deficiency is common, particularly inland areas where there is no access to marine foods. In iodine-sufficient areas, chronic autoimmune destruction of thyroid gland (e.g., Hashimoto’s thyroiditis) and iatrogenic causes from treatment of Graves’ disease are the leading causes of hypothyroidism (after thyroidectomy or radioactive iodine ablation). The prevalence of hypothyroidism increases with age, and the disorder is nearly 10 times more common in females than in males.1 Subclinical hypothyroidism is more prevalent than overt hypothyroidism in all age groups and can be seen in 4% to 15% of women, especially the elderly.2,3
Hypothyroidism occurs in 1% to 32% of patients taking amiodarone.1
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 thyroid-stimulating hormone from the pituitary gland or deficiency of thyrotropin-releasing hormone from the hypothalamus. Table 223-1 lists common causes of hypothyroidism. Euthyroid sick syndrome or low thyroxine syndrome, also called nonthyroidal illness, is the term used for patients with low triiodothyronine and thyroxine levels and a normal or low thyroid-stimulating hormone level, but who are clinically euthyroid. This condition is found in critically ill patients or those with severe systemic illness.
Table 223-1 Common Causes of Hypothyroidism |Favorite Table|Download (.pdf)
Table 223-1 Common Causes of Hypothyroidism
Primary Hypothyroidism (disorders of thyroid gland)
Secondary Hypothyroidism (disorders at hypothalamic-pituitary axis)
Autoimmune etiologies (Hashimoto’s)
Thyroiditis (subacute, silent, postpartum)*
Infiltrative causes (e.g., hemochromatosis, sarcoidosis)
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 of the brain)
- Drugs affecting thyroid gland function
- Potassium perchlorate
- Iodine (in patients with preexisting autoimmune disease)
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.
Symptoms can manifest in all organ systems and range in severity based on the degree of hormone deficiency (Table 223-2).
Table 223-2 Symptoms and Signs of Hypothyroidism
The common clinical features of hypothyroidism are listed in Table 223-2. Additional cardiopulmonary findings include angina, bradycardia, distant heart sounds from pericardial effusion, low voltage on the electrocardiogram, pleural ...