Subclinical thyroid disease

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