Cardiovascular disease and thyroid function.

Thyroid function has a profound effect on the heart, and both all-cause and cardiovascular mortality rates are increased in hyperthyroidism. New-onset atrial fibrillation carries a prolonged risk for the development of hyperthyroidism, suggesting altered availability of thyroid hormones at the cellular level. Subclinical hyperthyroidism is associated with increased left ventricular mass of the heart, which reverts after obtaining euthyroidism. Mortality and risk of major cardiovascular events are increased. Subclinical hypothyroidism is also associated with subtle changes in the heart, e.g. its increased stiffness, which reverts after treatment with levothyroxine. Mortality seems mildly reduced, although the risk of myocardial infarction is increased. The risk of atrial fibrillation is related to thyroid function over the whole spectrum: from a reduced risk in overt and subclinical hypothyroidism, a progressively increased risk in people with different levels of reduced TSH to a physiologically 'dose-dependent' effect of thyroid hormones on the heart in overt hyperthyroidism. Heart failure represents an intriguing clinical situation in which triiodothyronine treatment might be beneficial. In conclusion, subclinical dysthyroid states affect the heart with subsequent changes in morbidity and mortality. Subclinical hyperthyroidism seems a more serious condition than subclinical hypothyroidism, which should affect treatment decision in a more aggressive manner.