SUBCLINICAL HYPERTHYROIDISM

Subclinical hyperthyroidism is one of several causes of low serum thyr oid-stimulating hormone (TSH) concentrations. It is most simply define d as low serum TSH and normal serum thyroid hormone concentrations. Ot her causes of low serum TSH concentrations from which subclinical hype rthyroidism must be distinguished are nonthyroidal illness and seconda ry hypothyroidism. The most common cause of subclinical hyperthyroidis m is excessive thyroid hormone therapy. Other causes are an autonomous ly functioning thyroid adenoma, multinodular goiter, and Graves' disea se. Because many patients have no evident thyroid disease and because serum TSH concentrations may be normal weeks or more later, interventi on should not be seriously considered until the biochemical findings a re found to persist. The likelihood of progression to overt hyperthyro idism is low. Some patients have minor symptoms and signs of hyperthyr oidism, but most do not, Some may also have minor stimulation of cardi ac function, an increased risk of atrial fibrillation, or accelerated bone loss, Management should be based on the cause of the subclinical hyperthyroidism and on whether the patient has any clinical manifestat ions of thyroid hormone excess or underlying problems likely to be wor sened by small increases in thyroid secretion.