| Overt hyperthyroidism is relatively common in the general population. However, it has been quite difficult to quantify the prevalence of subclinical disease. The observed prevalence of subclinical hyperthyroidism varies regionally depending on dietary iodine intake 8 and the prevalence of thyroid autoantibodies; nonthyroidal illness, especially in institutionalized or elderly patients, may be difficult to differentiate from subclinical hyperthyroidism; and the sensitivity of TSH assays used in various studies has differed 9 . Some studies have excluded patients on L-thyroxine (L-T 4 ) therapy, a major cause of subclinical thyrotoxicosis, while others have not. One US study found suppressed TSH values in 2.5 % of the population over age 55 10 . The Whickham survey, a large English cross-sectional study begun in the 1970s, found that overt hyperthyroidism was present in 2% of adult women (10 times more common than in men)11 . Subclinical hyperthyroidism may be even more common than overt hyperthyroidism in older patients. In the 20-year follow up of the Whickham survey, 2–3 % of the cohort had a suppressed thyroid-stimulating hormone (TSH) without overt symptoms 12 . Data from the original Framingham Heart Study cohort, looking at 2575 ambulatory subjects over age 60, found suppressed TSH levels in 3.9 % (with half of those taking exogenous thyroid hormone), and overt hyperthyroidism in only 0.2 % 7 . More recently, a Colorado prevalence study found that subclinical hyperthyroidism was present in 2.1% of adults overall, but in 20 % of those taking thyroid medication 13 . Finally, the third National Health and Nutrition Examination Survey in the United States from 1988–1994 found overt hyperthyroidism in 0.5 % and subclinical hyperthyroidism in 0.8 % of the adult population ages 12 to greater than 80 years 14 . Of interest is the fact that both autoimmuine thyroid disease and mean serum TSH values were lower in African-Americans than in Caucasians. |
