Thyroid disorders affect 1 in 200 adults but are more common in women and with advancing age. The incidence of hypothyroidism, for instance, is 0.3-5 cases per 1000 individuals per year, including 7% of women and 3% of men aged 60-89 years. Hypothyroidism is much more common than hyperthyroidism, nodular disease, or thyroid cancer. Thyroid nodules occur in 4%-8% of all individuals and, like other thyroid problems, increase in incidence with age.
Thyroid disease is more common in people who have conditions such as diabetes or other autoimmune diseases (eg, lupus); in those with a family history of thyroid disease or a history of head and neck irradiation; and in patients who use certain medications, including amiodarone and lithium. Recent guidelines from the American Thyroid Association suggest that all adults have their serum thyroid-stimulating hormone (TSH) concentrations measured, beginning at age 35 and every 5 years thereafter.
Causes of hypothyroidism are outlined in Table 36-1. The most common noniatrogenic condition causing hypothyroidism in the United States is Hashimoto thyroiditis. Other common causes are post–Graves disease, thyroid irradiation, and surgical removal of the thyroid. Hypothyroidism may also occur secondary to hypothalamic or pituitary dysfunction, most commonly in patients who have received intracranial irradiation or surgical removal of a pituitary adenoma. In addition, some patients may have mild elevations of TSH despite normal thyroxine levels, a condition termed subclinical hypothyroidism.
Table 36–1. Causes of Hypothyroidism. |Favorite Table|Download (.pdf)
Table 36–1. Causes of Hypothyroidism.
|Primary Hypothyroidism (95% of cases)|
|Idiopathic hypothyroidism (probably old Hashimoto thyroiditis)|
|Late-stage invasive fibrous thyroiditis|
|Drugs (lithium, interferon)|
|Infiltrative diseases (sarcoidosis, amyloid, scleroderma, hemochromatosis)|
|Secondary Hypothyroidism (5% of cases)|
|Pituitary or hypothalamic neoplasms|
|Pituitary necrosis (Sheehan syndrome)|
Patients with hypothyroidism present with a constellation of symptoms that can involve every organ system. Symptoms include lethargy, weight gain, hair loss, dry skin, slowed mentation or forgetfulness, depressed affect, cold intolerance, constipation, hair loss, muscle weakness, abnormal menstrual periods (or infertility), and fluid retention. Because of the range of symptoms seen in hypothyroidism, clinicians must have a high index of suspicion, especially in high-risk populations. In older patients, hypothyroidism can be confused with Alzheimer disease or other conditions that cause dementia. In women, hypothyroidism is often confused with depression.
Physical findings that can occur with hypothyroidism include low blood pressure, bradycardia, nonpitting edema, generalized hair thinning along with hair loss in the outer third of the eyebrows, skin drying, and a diminished relaxation phase of reflexes. The thyroid gland in a patient with chronic thyroiditis may be enlarged, atrophic, or of normal size. Thyroid nodules are common in patients with Hashimoto thyroiditis.