Subclinical hyperthyroid and hypothyroid disease are laboratory diagnoses. There is little evidence that early treatment alters the clinical course. A serum thyroid-stimulating hormone level of less than 0.1 μU per mL is associated with progression to overt hyperthyroidism, atrial fibrillation, reduced bone mineral density, and cardiac dysfunction. There is insufficient evidence that treatment of subclinical hypothyroidism is beneficial. Patients with a serum thyroid-stimulating hormone level greater than 10 μU per mL have a higher incidence of elevated serum low-density lipoprotein cholesterol concentrations however, evidence is lacking for other associations. There is good evidence that subclinical hypothyroidism is associated with progression to overt disease. Most national organizations recommend against routine screening of asymptomatic patients, but screening is recommended for high-risk populations. Subclinical hyperthyroidism is found in approximately 2 percent of the population. The prevalence of subclinical hypothyroidism is about 4 to 8.5 percent, and may be as high as 20 percent in women older than 60 years. The management of subclinical thyroid dysfunction is controversial. Subclinical thyroid dysfunction is defined as an abnormal serum thyroid-stimulating hormone level (reference range: 0.45 to 4.50 μU per mL) and free thyroxine and triiodothyronine levels within their reference ranges.
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