Pericardial effusion in primary hypothyroidism. Pericardial effusion is reported to occur in 30% to 80% of subjects with hypothyroidism. However, these earlier studies were conducted when the diagnosis of hypothyroidism was only suspected and was confirmed only in the presence of classic clinical features. In contrast, the diagnosis has recently been established in the early mild stage or more often in an asymptomatic stage because of more frequent or routine determinations of thyroid function tests, especially in the elderly. Thus the subjects in the older studies were severely hypothyroid at the time of diagnosis and may not be representative of the present hypothyroid population. For this reason, 30 subjects with hypothyroidism were evaluated with echocardiography to reassess the evidence of pericardial effusion in this disorder. Only two subjects demonstrated pericardial effusion, and in only one of them with severe disease could the pericardial effusion be attributed to hypothyroidism, since it resolved on the patient's attaining the euthyroid state. Thus the incidence of pericardial effusion was only 3% to 6%, depending on the inclusion of one or both subjects, an extremely infrequent occurrence when compared with that of previous studies. Moreover, the occurrence of pericardial effusion in hypothyroidism appears to be dependent on the severity of the disease. Thus pericardial effusion may be a frequent manifestation in myxedema, an advanced severe stage, as previously found, but a rare association of hypothyroidism, an early mild stage, because of the timeliness with which the latter condition is nowadays detected. 