Objectives: To determine the clinical correlates of peri- cardial effusion (PE) and a low-voltage electrocardiogram (ECG) in patients with primary hypothyroidism. Methods: ECG, echocardiography and blood tests, including thyroid function tests, were performed in 64 consecutive patients with primary hypothyroidism. Results: Twenty-four patients (38%) had PE and 16 patients (25%) had low voltage. To determine the important variables present in patients with hypothyroidism that may be related to PE, 3 variables (free T4, pulmonary artery systolic pressure and serum albumin) were used in the multivariate analysis. From the analysis, free T4 (odds ratio = 0.032) and pulmonary artery systolic pressure (odds ratio = 1.085) emerged as significant variables related to PE, whereas when 4 variables (free T4, serum albumin, pulmonary artery systolic pressure and PE) were used to determine the important variables related to low voltage, free T4 (odds ratio = 0.029) and serum albumin (odds ratio = 0.255) were the significant variables, but PE had no significant relation with low voltage (p = 0.424). Conclusions: In addition to its association with more severe thyroid hormone deficiency, the presence of PE was also associated with an increase in pulmonary artery pressure, whereas attenuation of QRS voltage was associated with lower colloid osmotic pressure in patients with hypothyroidism.