Subclinical hyperthyroidism and atrial fibrillation.

Subclinical hyperthyroidism is characterized by a clearly low serum concentration of thyrotropin (TSH) and the absence of obvious symptoms of hyperthyroidism. Whether or not all persons with a low value for serum TSH can be considered subclinically hyperthyroid is uncertain, but the low serum TSH per se is a risk factor for atrial fibrillation and perhaps other cardiovascular disease. Screening all persons, even in the older age groups, for subclinical hyperthyroidism may not be justified in and of itself but such persons would likely be screened for subclinical hypothyroidism and so those with subclinical hyperthyroidism identified. There are no controlled trials that show the benefit of treatment of subclinical hyperthyroidism, thus therapy at present needs to be determined by clinical judgement, aided by monitoring the patient to see if the low serum TSH level persists and assessing the presence or absence of other determinants of therapy such as an elevated serum triiodothyronine (T3) concentration, a multinodular goiter, or the appearance of overt symptoms.