[Subclinical thyroid disease]

Subclinical thyroid disease is defined as a disorder involving an abnormal serum thyroid-stimulating hormone level (TSH), but free thyroxine (FT4) and triiodothyronine (FT3) levels within the reference range. The prevalence of subclinical hypothyroidism is about 4 to 8.5 percent of the population, and may rise to about 20 percent in women older than 60 years. The clinical significance of subclinical thyroid disease remains controversial. Several studies have reported that subclinical hypothyroidism may be a risk factor for atherosclerosis and cardiovascular disorders, and subclinical hyperthyroidism for atrial fibrillation and reduced bone mineral density. However, there is a little evidence showing the significance of early treatment for both disorders. In this review, we discuss the prevalence, diagnosis, natural history, and potential pathophysiological consequences of subclinical hypo- and hyperthyroidism.